feat: worktree lifecycle hardening (orphan cleanup, lock detection)

[dougss]

Summary

Harden worktree management: cleanup orphans on startup, detect lock files, check merge conflicts before PR.

Current State

• src/integrations/git/worktrees.ts handles creation
• Config has orphan_worktree_cleanup_on_startup but needs robust implementation
• No lock detection or proactive conflict checking

Plan

1. cleanupOrphanWorktrees() — scan worktrees_dir, remove untracked in DB
2. detectWorktreeLock(repoPath) — check .git/worktrees/*/locked
3. checkMergeConflicts(repoPath, target) — git merge-tree dry-run
4. Call on daemon startup, before provisioning, and before PR
5. Emit new event types

Files

• src/integrations/git/worktrees.ts
• src/core/pipeline.ts
• src/harness/events.ts