fix(terminal): make DECSET 2031 subscriptions silent (#13904)

fish arms `CSI ?2031h` before painting each prompt and withdraws it when it
hands the tty to a child — a ~1ms window. Orca answered that subscribe with
`CSI ?997;Nn` across a 1-3ms renderer hop, so the reply landed after the
withdrawal and was read as stdin by the next child, corrupting `brew`/`npx`
`[y/N]` prompts.

The reply is not stale by Orca's own view when written (measured
staleReplies: 0), so no suppress-the-stale-reply scheme can close this — the
information needed to suppress does not exist yet. Nothing asked for the reply
either. The Contour spec says a terminal "should only send out the DSR when the
palette has been updated"; Ghostty (Termio.zig:729 — force=true reachable only
from the ?996n DSR), iTerm2 (VT100Terminal.m:995 — flag only) and xterm.js
(InputHandler.ts:2035 — flag only) all emit nothing on the DECSET. So stop
entering the race: record the subscription, answer nothing.

Of 17 real programs measured under a pty, only fish, tmux, claude and opencode
subscribe; none block on a reply, and answering produces one redundant palette
re-query and zero rendering difference. tmux is the only one that sends `?996n`,
which Orca still answers.

- Subscribes are record-only at all four emitters (live scan, hidden-gate fact,
  parked byte watcher, parked responder — the last is deleted, it only replied).
- `?996n` answers, the subscription registry, and the theme-flip push are
  unchanged. `paneLastThemeMode` is still seeded at subscribe so the next
  appearance re-apply is not read as a flip.
- Replay grammar carries `?2031l` alongside `?2031h`, so a late-attaching remote
  client no longer registers a subscription the TUI already retired.

Also closes fish-integration gaps found alongside: `unset` (which fish lacks)
becomes `set -e` on paths parsed by the client's login shell, `config.fish` is
parsed for agent-home detection, and bracketed-paste startup delivery is made
consistent across local/daemon/relay.

Regression test drives real fish 4.7.1 under node-pty and asserts on what the
child process reads; it fails against pre-fix code with the exact payload from
the issue. CI installs fish 4 and fails loudly rather than skipping.

Closes #9993

Co-authored-by: Orca <help@stably.ai>

.github/workflows/pr.yml

@@ -176,6 +176,12 @@ jobs:
   shell_contracts:
     name: shell contracts
     runs-on: ubuntu-latest
+    env:
+      # Why: the suites below gate their live fish tests on the binary, which is
+      # right on a developer machine and wrong here — this job is a required check
+      # and its fish lane is the only end-to-end guard for #9993, so a skip would
+      # report green with nothing exercised. Turns those skips into failures.
+      ORCA_REQUIRE_FISH: '1'
 
     steps:
       - name: Checkout
@@ -186,8 +192,46 @@ jobs:
       # Why fish: shell-ready.test.ts gates its live fish test on the binary being
       # present, so without this the fish barrier is only covered by config-shape
       # assertions and never actually exercised.
+      # Why release-4: DECSET 2031 arming lives in the fish 4.0 Rust tty_handoff, and
+      # fish-color-scheme-child-stdin.node-pty.test.ts (#9993) needs it. Noble ships
+      # 3.7, so the PPA is what makes that lane real.
       - name: Install zsh and fish
-        run: sudo apt-get update && sudo apt-get install -y zsh fish
+        run: |
+          # Why the update/PPA/fish steps are tolerant: a repo the runner image already
+          # ships can lack a Release file for this suite, and a failed add-apt-repository
+          # still leaves its list entry behind — either makes `apt-get update` exit
+          # non-zero and would red this required check over something unrelated to the
+          # PR. Every fish outcome is judged by the version gate below instead, so only
+          # the zsh install (which has no such gate) stays fatal here.
+          sudo apt-get update || true
+          # Why retry only here: adding the PPA is the network-flaky step, and the
+          # version gate below is fatal, so a transient Launchpad blip would
+          # otherwise red a required check on PRs unrelated to shells.
+          for attempt in 1 2 3; do
+            sudo add-apt-repository -y ppa:fish-shell/release-4 && break
+            echo "add-apt-repository attempt ${attempt} failed; retrying" >&2
+            sudo add-apt-repository -y -r ppa:fish-shell/release-4 || true
+            sleep 5
+          done
+          sudo apt-get update || true
+          # Why both shells on one line: pr-workflow-parallelism.test.mjs parses only the
+          # first install command in this step to prove the lane really installs them.
+          sudo apt-get install -y zsh fish
+
+      # Separate from the install so the failure names the contract, not an apt error.
+      # ORCA_REQUIRE_FISH re-checks this at test time; this step just fails in seconds
+      # instead of after a full dependency install.
+      - name: Require fish 4+
+        run: |
+          version="$(fish --version 2>/dev/null || true)"
+          major="${version##*version }"
+          major="${major%%.*}"
+          case "$major" in '' | *[!0-9]*) major=0 ;; esac
+          echo "${version:-<fish not installed>}"
+          if [ "$major" -lt 4 ]; then
+            echo "::error::shell contracts needs fish 4+ (DECSET 2031 arming, #9993) but got '${version:-none}'. Fix the ppa:fish-shell/release-4 install rather than letting the fish lane skip." >&2
+            exit 1
+          fi
 
       - uses: ./.github/actions/install-node-dependencies
         with:
@@ -201,6 +245,7 @@ jobs:
             src/main/providers/local-pty-shell-ready.test.ts \
             src/main/providers/__tests__/shell-ready-framework-example.test.ts \
             src/main/pty/omp-shell-wrapper.node-pty.test.ts \
+            src/renderer/src/components/terminal-pane/fish-color-scheme-child-stdin.node-pty.test.ts \
             src/shared/posix-command-path-lookup.test.ts
 
   test:
@@ -235,6 +280,7 @@ jobs:
             --exclude=src/main/providers/local-pty-shell-ready.test.ts \
             --exclude=src/main/providers/__tests__/shell-ready-framework-example.test.ts \
             --exclude=src/main/pty/omp-shell-wrapper.node-pty.test.ts \
+            --exclude=src/renderer/src/components/terminal-pane/fish-color-scheme-child-stdin.node-pty.test.ts \
             --exclude=src/shared/posix-command-path-lookup.test.ts \
             --exclude=tests/e2e/cross-version-wire/** \
             --shard=${{ matrix.shard }}/${{ matrix.shard_total }}