fix: Polecat lifecycle cleanup - stale worktrees and git tracking

Fixes gt-v07fl: Polecat lifecycle cleanup for stale worktrees and git
tracking conflicts.

Changes:
1. Add .claude/ to .gitignore (prevents untracked file accumulation)
2. Add beads runtime state patterns to .gitignore (prevents future tracking)
3. Remove .beads/ runtime state from git tracking (mq/, issues.jsonl, etc.)
   - Formulas and config remain tracked (needed for go install)
   - Created follow-up gt-mpyuq for formulas refactor
4. Add DetectStalePolecats() to polecat manager for identifying cleanup candidates
5. Add CountCommitsBehind() to git package for staleness detection
6. Add `gt polecat stale <rig>` command for stale polecat detection/cleanup
   - Shows polecats without active sessions
   - Identifies polecats far behind main (configurable threshold)
   - Optional --cleanup flag to auto-nuke stale polecats

The existing `gt polecat gc` command handles branch cleanup.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -17,6 +17,9 @@
 .DS_Store
 Thumbs.db
 
+# Claude Code local state
+.claude/
+
 # Test
 coverage.out
 *.test
@@ -29,7 +32,17 @@ gt
 # Runtime state
 state.json
 .runtime/
+
+# Beads runtime state (not tracked)
+# Formulas ARE tracked for `go install @latest` - see bottom of file
 .beads/redirect
+.beads/issues.jsonl
+.beads/interactions.jsonl
+.beads/metadata.json
+.beads/mq/
+.beads/last-touched
+.beads/daemon-*.log.gz
+.beads-wisp/
 
 # Clone-specific CLAUDE.md (regenerated locally per clone)
 CLAUDE.md