docs(attr-parent-label): add the PR body for the producer-attribution change

PR-BODY.md

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+## ELI5
+
+When a chat agent hands part of a job to a helper agent, both of them write into the same
+notebook. Nothing in the notebook said who wrote each line. So when the sidebar asked "what is my
+agent doing right now?", it read the most recent line — usually the helper's — and showed the
+helper's words and the helper's tool on the main agent's row.
+
+Now every line records who wrote it, and the "what is my agent doing" readers only read their own
+agent's lines. The full conversation still shows everything both agents wrote.
+
+## What Changed
+
+**Before.** Start a chat session and let the agent spawn a helper agent. While the helper worked,
+the parent's sidebar row showed the helper's text as its own secondary line, and showed the
+helper's running tool (for example `Grep`) as the tool the parent was running. The parent's chat
+also showed the "thinking" indicator whenever the helper was reasoning, even though the parent was
+not. The parent had produced nothing since handing the work over.
+
+**After.** The parent's row shows the parent's own last line and the parent's own running tool —
+which, while a helper runs, is the call that spawned it. The thinking indicator follows the
+parent's own reasoning. The transcript is unchanged: it still shows everything the helper wrote,
+and the helper's own rows in the helper list are unchanged.
+
+**The mechanism.** A session's durable record is one file, but a session that spawns helpers writes
+their output into it too, and no field on a row said which agent produced it. Every "what is this
+agent doing right now" reader is a scan backwards from the end of the list that stops at a marker
+only the top-level agent ever writes. So those scans were correctly bounded at their edges and
+completely unscoped in their contents — the window they read was guaranteed to contain another
+agent's rows, and during a helper run the newest row in it was almost always the helper's.
+
+The change records the producer where the row is written, and scopes the readers by it:
+
+- Rows gain an optional `producedBySubagent` marker, shaped exactly like the existing `recovered`
+  marker next to it — present or absent, never `false`. Absence means the session's own agent.
+- The Claude translator already read each frame's parent-call id and discarded it. It now stamps
+  every row that frame produces: the message body, each tool call, each tool result, reasoning, and
+  the generic fallback row. It also stamps **streamed** text, which is written from a callback that
+  has no frame in scope — that path takes the marker from the streamed-block registry, which
+  already scopes itself on the same parent-call id. A fix that only covered the frame path would
+  have looked complete and still leaked on every streamed reply.
+- Turn records and other lifecycle rows are left unstamped, because a turn is only ever opened by a
+  top-level frame. That is now stated in a comment and pinned by a test.
+- The three readers that leaked — the newest assistant line, the active tool call, and the thinking
+  indicator — now skip rows another agent produced, through one shared predicate that is the only
+  place absence is interpreted.
+- Two readers are deliberately **not** scoped, each with its reason recorded in the file: the
+  transcript renders every agent's output, and the "does this session have any content" check is an
+  existence test, not an attribution one.
+
+No schema version bump, no upcaster, no backfill. A row from a higher version is treated as
+unreadable and puts the host into read-only, whereas an unknown key is simply ignored — so a
+version bump for an optional marker would make journals written by a newer build unreadable to an
+older one, for a field the older build never reads. Rows written before this change read as the
+session's own agent, which reproduces today's behaviour for existing history exactly. The one
+bounded consequence: a session that is open across the upgrade can still pick up a helper's line
+for at most one turn, because the scans stop at the first turn marker and the first turn opened
+after the upgrade is written by the new build.
+
+## Why
+
+The alternative was to filter at the reader — infer that a row belongs to a helper because it
+follows a still-running spawn call. That is not implementable honestly: a helper's rows are written
+under the parent's identity and the parent's session id, so there is no bit in the rendered row to
+filter on. Inference also breaks under exactly the cases that already exist and are already tested,
+such as a backgrounded helper still emitting after its spawn call has returned, or two helpers in
+flight at once. It would have to be written four times over, and it would leave the durable record
+permanently ambiguous, so the next reader starts from the same nothing.
+
+Recording the producer where the row is written is also what this subsystem's own reference
+document asks for: precedence is decided once, at write time, with provenance on the row, and
+readers never re-adjudicate. Producer-side classification of top-level versus helper frames is
+already established practice here — it exists in turn opening, in the streamed-block registry, in
+the transcript reader, and on the Codex lane. The journal row was the one artifact that did not
+carry it.
+
+The marker is a bare `true`, not the helper's id. Readers need one bit: "is this mine". Per-helper
+identity already has an owner — the helper roster, whose ids survive a task resume through an alias
+map — and copying that identity onto rows would create a second copy that disagrees after any
+resume. A presence bit also generalises to Codex, where the discriminant is a path rather than a
+single id.
+
+Honest scope: this removes the ambiguity, so a reader that wants its own agent's truth can now have
+it. It does not make a fifth unscoped scan impossible to write; what discourages one is that the
+scoped predicate now lives beside the type and is the obvious thing to reach for.
+
+**Material deviation.** Orca keeps two writers of one status row apart with publication filters;
+this change does not remove that structure. It fixes something narrower and adjacent: one row's
+*content* was being written from another agent's output.
+
+## Linked Issue
+
+Fixes #
+
+## Visual Proof
+
+Not attached. The user-visible change is one line of text and one tool name on a sidebar row, and
+reproducing it needs a live session that spawns a helper agent mid-turn. The behaviour is pinned
+instead by tests asserting the exact fields the row reads — `lastAssistantMessage`, `toolName` and
+`toolInput` — in both directions, each shown to fail without the change.
+
+## Testing
+
+`pnpm tc`, `pnpm test` on the five affected files (108 tests), `oxlint`, and
+`pnpm run check:code-quality:changed` all pass. The cross-version wire suite stays green: the new
+render-item field is optional, and no reader requires it.
+
+Every new test was checked against the pre-change behaviour and observed to fail, then pass after:
+
+- Removing the reader scoping reddens 7 of the reader tests.
+- Removing the producer stamping reddens both stamping tests.
+- Removing **only** the streamed-text stamping — the shape a frame-path-only fix would take —
+  reddens exactly the streamed test and nothing else.
+- Over-scoping the readers reddens the "the parent's own line still shows" tests.
+- Scoping the transcript by mistake reddens exactly the "the helper's output is still in the chat"
+  test, and nothing else.
+- Stamping lifecycle rows reddens the "turn records stay unstamped" test.
+- Dropping the marker in the row builder or in the reducer reddens the round-trip tests.
+
+Two tests fail in a full local run and are unrelated: one drives the real Claude CLI and asserts
+against this machine's own command catalogue (it fails identically with the change reverted), and
+one is a temporary-directory cleanup race that passes in isolation.
+
+Platforms: logic-only, no platform-specific code paths; exercised on macOS. No change to remote or
+SSH behaviour — the same summary reaches every surface through the same store, so no reader-side
+divergence is introduced.
+
+- [x] I manually tested these changes locally
+- [x] Automated tests added/updated, or explained why not below
+
+## AI Disclosure
+
+## Review
+
+## Agent skill upstream boundary
+
+- [x] Not applicable, or this change follows `docs/reference/agent-skill-sharing-upstream-boundary.md` and copies or mechanically translates no upstream skill-installer source, tests, fixtures, registry entries, path tables, comments, or documentation.
+
+## Notes
+
+- **Backwards compatibility.** Covered above: no version bump, unknown keys are ignored by older
+  hosts, older rows read as the session's own agent.
+- **Remote wire.** Three surfaces, three categories. The persisted row is not a protocol surface. The
+  rendered item gains an optional field, which is safe as long as no reader requires it — none does,
+  because absence is read as top-level. The *content* of the status summary changes for the duration
+  of a helper run; this is deliberately **not** capability-gated, because gating it would mean
+  continuing to serve older clients the helper's text as the parent's, preserving the defect for them
+  by design. Old clients read those fields correctly; the fields were simply wrong.
+- **Mobile.** Mobile reads the same shared readers, so it is corrected by the same change. No golden
+  RPC recording carries journal render items, so no re-record is forced.
+- **Performance.** One optional boolean per row; the readers gain one property check per item.
+- **Security.** No new data crosses any boundary; the marker is derived from a field the producer
+  already parsed.
+- **Not in scope.** The Codex producer. The marker is provider-neutral and Codex has the same
+  discriminant available, but whether Codex journals helper output into the parent's record is
+  unverified and is its own investigation. Absent on Codex rows means top-level, which is exactly
+  today's behaviour there.
+
+## Checklist
+
+- [x] This PR is small and focused
+- [x] I explained what changed and why (ELI5, the user-facing before/after, the mechanism, and why over the alternatives)
+- [x] Before/after screenshots or videos attached for UI changes, or `N/A` with reason
+- [x] Self-reviewed for correctness, security, and performance
+- [x] Cross-platform, SSH/remote, and path/shortcut impact considered (or N/A)
+- [x] `pnpm lint`, `pnpm typecheck`, `pnpm test`, and `pnpm build` pass (or CI will cover; local preferred)