Bug: Composer agent hangs indefinitely during issue decomposition (no timeout enforcement)

[randomm]

What happened

The Composer agent hangs indefinitely when decomposing GitHub issues into tasks. In testing, Composer ran for 20+ minutes without creating any tasks, leaving the job in "running" state with Pulse PID showing "none".

Expected behaviour

Composer should decompose issues within 2-3 minutes and either:

• Successfully create tasks and hand off to Pulse
• Time out with an error message after 5 minutes (per composer.ts:28)

Root Cause Analysis

Location: packages/opencode/src/tasks/composer.ts:51-54

The timeout mechanism exists but has a critical gap:

await Promise.race(
  SessionPrompt.prompt({ sessionID: session.id, agent: "composer", ... }),
  timeoutPromise,  // 5 minutes
)

Problem: The prompt() call blocks the entire event loop while the LLM processes. The 60+ line Composer prompt with 15+ rules may cause the LLM to take 5+ minutes just to process context before responding. The timeout fires but cannot interrupt an active prompt() call mid-execution.

Secondary issue: SessionPrompt.cancel() deletes session state, but the while(true) loop in prompt.ts:309 may not check abort.aborted until the next iteration, creating a race condition.

Steps to reproduce

1. Create a moderately complex GitHub issue (e.g., test(taskctl): add utility function to validate branch name safety #393 with TDD requirements, acceptance criteria, test scenarios)
2. Run taskctl start 393
3. Observe: Job status shows "running" but no tasks created after 5+ minutes
4. Pulse PID remains "none" (Pulse never starts because Composer hasnt finished)

Environment

• Branch: dev (randomm/opencode fork)
• Version: Current HEAD (commit 3277b60)
• Tested with: Issue test(taskctl): add utility function to validate branch name safety #393 (branch validation utility task)

Acceptance Criteria

• Composer times out after 5 minutes with clear error message
• Job transitions to "failed" state with actionable error (not stuck in "running")
• PM session receives notification: "Composer timed out for issue #N"
• Add instrumentation logging: "Composer started at [timestamp]", "Composer timed out at [timestamp]"
• Document in lievo/ or code comment: "Composer prompt should be < X tokens to avoid timeout"

Definition of Done

• Timeout enforcement verified with test (mock slow LLM response)
• No regression: normal Composer runs still complete successfully
• Error message includes issue number and suggestions (e.g., "Simplify issue description")
• Code review passed (security, type safety, architecture lenses)