fix(spool): classify detached lanes waiting on their own background subagent as failed

[agents-squads]

Closes #1130.

A detached lane's terminal result event can report a clean, non-error
success while the lane's own last turn was still waiting on a background
Task/Agent subagent it spawned and never got a result back from ("I'll pause
here until it completes"). In a detached lane there is no next turn to
resume that wait — the run gets recorded as a zero-deliverable "completed"
forever (repro'd twice on 2026-07-14/15, see issue).

This is a distinct terminal-state class from #1123's deliver-and-stop CI
gate (which verifies CI on a PR the lane already believes it produced) —
here the lane never produced anything to gate.

Fix, built on the same seam #1131 added for "claude's own terminal
state can lie":

• stream-json.ts now correlates Agent/Task tool_use ids against later
  tool_result blocks (the same shape exec-events.ts already uses for the
  obs events file) and exposes the count left unresolved by the time the
  stream ends as openBackgroundSubagents.
• spool.ts's reconcile (toRecord) adds a third branch to the existing
  #1131 reclassification chain: a sawResult && !isError run is
  reclassified to failed when openBackgroundSubagents > 0 and the
  run produced zero commits/PRs/issues. A subagent that resolved before the
  stream ended, or a run that still delivered a commit/PR/issue despite a
  dangling background task, are both left completed.

Structural, not text-matching — the issue's own follow-up comment notes a
second occurrence phrased the "waiting" differently ("waiting for the
automatic notification" vs. "I'll pause here"), so regexing the model's
free-form text would just chase rewordings. Whether a synchronous subagent
call resolved before the parent turn ended is a hard fact of the stream.

Test plan

• npm run typecheck — pass
• npm run build — pass
• npm test — 2365/2365 pass (145 files), including new cases:
  • test/stream-json-parser.test.ts: open/resolved/ignored subagent tool_use tracking
  • test/spool.test.ts: the exact repro (clean result + hanging subagent + zero deliverable → failed), resolved-subagent stays completed, delivered-PR-with-dangling-subagent stays completed, legacy plain-text logs untouched
• npm run lint — 0 errors (pre-existing warnings only, unrelated to this change)

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/lib/stream-json.ts	95.45%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!