fix(native-chat): retry not-yet-flushed transcripts instead of settling into a permanent error (#8418)

* fix(native-chat): retry not-yet-flushed transcripts instead of settling into a permanent error

A freshly-created session's transcript .jsonl lands on disk seconds to
minutes after the process starts. Native chat's one-shot read raced that
first flush: a miss became a permanent "No transcript found" error and
the live-tail subscription silently degraded to a no-op, so the pane
never recovered even after the file appeared.

- transcript-reader/read-cache: mark the miss with notFound so callers
  can tell "not flushed yet" from a real parse/IO error (never cached).
- transcript-watch: poll resolve+install (500ms backoff, 5s cap) for the
  subscription's lifetime instead of returning a dead no-op watcher.
- use-native-chat-live-session: retry a notFound read with backoff for
  up to 60s while staying in the loading state, and let live appends
  render over a stale initial-read error.

Fixes #8401

Claude-Session: https://claude.ai/code/session_01HA5g3X7wCakBttBpDru9Fp

* fix(native-chat): address CodeRabbit review — ENOENT retryable, content over spinner, blank-id guard, unref poll timer

- transcript-reader: an ENOENT after a successful resolve is the same
  first-flush/rotation race as an unresolved path — mark it notFound.
- use-native-chat-live-session: live appends landing mid-retry render
  instead of the loading state (mirrors the stale-error gate).
- transcript-watch: bail out for a blank session id with no explicit
  file (nothing to resolve-poll), and unref the poll timer so headless
  serve shutdown is never held open by an unresolvable session.

Claude-Session: https://claude.ai/code/session_01HA5g3X7wCakBttBpDru9Fp

---------

Co-authored-by: kaynan <kaynan.camargo@terceiro-sky.com.br>

src/main/native-chat/transcript-read-cache.ts

@@ -91,7 +91,9 @@ export async function readNativeChatTranscriptCached(
 ): Promise<ReadTranscriptResult> {
   const filePath = await resolveSessionFilePath(agent, sessionId, { transcriptPath })
   if (!filePath) {
-    return { error: `No transcript found for ${agent} session ${sessionId}` }
+    // Not cached (see below): a not-yet-flushed transcript should be re-checked
+    // on the next call, not pinned as a settled miss (#8401).
+    return { error: `No transcript found for ${agent} session ${sessionId}`, notFound: true }
   }
 
   const key = cacheKey(agent, filePath)