fix(codex-cli): report spawn timeouts as timeouts, not exit code -1

[annieyii]

Closes #718.

When the Codex CLI spawn timeout fires, users saw Codex CLI exited with code -1: followed by the process's stderr. -1 is a sentinel codex_cli.rs
invents, not something Codex returns, so the message named a crash that never
happened. The sentence that did name the cause was appended to the tail of
stderr, behind up to STDERR_LIMIT_BYTES of unrelated startup output, so it was
effectively invisible.

Changes

src-tauri/src/commands/codex_cli.rs

• done_payload() emits timedOut and timeoutMinutes as their own fields.
  code now carries whatever the child reported instead of being overwritten
  with -1.
• The timeout sentence is no longer appended to stderr, since the fact is
  structured now. A consequence worth naming: [stderr truncated] was
  suppressed on timeouts by the else if that carried that append, and now
  applies to them too.

src/lib/codex-cli-transport.ts

• timedOut is checked ahead of both existing branches. This ordering is
  load-bearing rather than cosmetic: the timeout task removes the child handle
  in order to kill it, so code is null here on every platform. The
  exit-code branch skips null outright, and the branch after it calls
  onDone once any agent message has arrived, so a timeout that had already
  produced partial output would be reported to the user as a success.
• The message leads with the cause and keeps the captured diagnostic below it,
  under a heading naming the stream it actually came from. It states only what
  was observed, that LLM Wiki did not see the run report completion and stopped
  waiting, rather than that the CLI crashed or that the kill succeeded: the
  child can exit on its own just before the deadline and still be marked timed
  out.

Scope and limits

• code is None on the timeout path because the handle was taken away, not
  because a status was observed. start_kill()'s result is still unchecked;
  this change does not try to fix that, it stops the message from claiming
  otherwise.
• The Rust test pins the emitted key names. The TypeScript side reads them by
  hand and can still drift independently; nothing here exercises a real Tauri
  emit end to end.
• A cancel and a timeout can still race at the deadline. Behavior is unchanged
  from before this patch, where the same race produced code -1.
• Codex CLI detection is untouched. fix(codex-cli): tolerate slow cold-start version detection #711 adjusts the codex --version timeout
  and applies cleanly over this change; the two do not overlap.

Tests

src/lib/codex-cli-transport.test.ts, 17 passing. Three added:

• A timeout leads with the cause and keeps unrelated stderr below it.
• A timeout that already produced a partial agent message errors rather than
  calling onDone. This is the regression the ordering exists to prevent; it
  fails if the timedOut check moves back below the others.
• Fallback output is labelled stdout when stderr is empty.

src-tauri/src/commands/codex_cli.rs, 9 passing, 2 added over done_payload().
Mutating timedOut to timed_out turns both red; before this test that typo
would have shipped with the whole suite green.

Both the ordering assertion and the key-name assertion were mutation-tested by
reverting the behaviour they guard and confirming a named test goes red.

Verification

npm run build, npm run test:mocks (132 files / 1879 tests), npm run mcp:test (22/22), cargo build, cargo test, cargo fmt --check,
git diff --check.