fix: improve worker lifecycle reliability

[whitmo]

Summary

• Detect stale PID=0 workers: Workers where Claude failed to start had PID=0 and were never detected as dead by the health check. Now marks them for cleanup after a 2-minute grace period.
• Clean up workers with dead processes: When a worker's process dies but its tmux window still exists, health check now marks non-persistent agents for cleanup instead of silently ignoring.
• Rollback on creation failure: If createWorker() fails after creating worktree/tmux window (e.g., daemon registration fails), orphaned resources are now cleaned up.
• Record task history on worker rm: Manual worker removal via worker rm now records task history, matching automatic cleanup behavior.

Context

This addresses the P0 roadmap item "Reliable worker lifecycle - Workers should start, complete, and clean up without manual intervention." The audit identified four gaps in the worker lifecycle that could require manual intervention:

1. checkAgentHealth() skipped PID=0 workers entirely (line 338: if agent.PID > 0)
2. Dead worker processes were logged but not cleaned up if tmux window still existed
3. createWorker() had no rollback - partial failures left orphaned tmux windows and worktrees
4. handleRemoveAgent() didn't call recordTaskHistory() unlike cleanupDeadAgents()

Test plan

• New test: TestCheckAgentHealthPIDZeroStaleWorker - verifies PID=0 grace period logic
• New test: TestHandleRemoveAgentRecordsTaskHistory - verifies task history is recorded on manual removal
• All existing tests pass (go test ./... - 27 packages)
• Build succeeds (go build ./cmd/multiclaude)

Opportunities (not implemented)

• Add retry backoff for persistent agent auto-restart to avoid spam on repeated failures
• Improve tmux session restoration to not nuke all agents on transient failure
• Add a health check diagnostic endpoint showing per-agent status

🤖 Generated with Claude Code

[whitmo]

Triage Review

Priority: P0 (Reliable worker lifecycle - roadmap item)
Build: Pass
Tests: Pass (new tests: TestCheckAgentHealthPIDZeroStaleWorker, TestHandleRemoveAgentRecordsTaskHistory)
Merge conflicts: Conflicts with PR #364 in internal/cli/cli.go (both modify createWorker)
Roadmap alignment: Directly addresses P0 "Reliable worker lifecycle" item

Changes:

• Detects stale PID=0 workers after 2-min grace period
• Cleans up dead worker processes
• Adds rollback on worker creation failure
• Records task history on manual worker rm

Recommendation: Merge before #364. Solid fix with good test coverage.

[whitmo]

Local CI Verification (2026-03-12)

Check	Result
go build	PASS
go vet	PASS
go test ./...	PASS (all 22 packages)

CI Status: No GitHub Actions checks are running — this is expected for first-time fork PRs. GitHub requires a maintainer to approve workflow runs for PRs from forks.

Branch is rebased on upstream/main (0 commits behind). Ready for maintainer review and CI approval.