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>

internal/git/git.go

@@ -699,6 +699,24 @@ func (g *Git) CommitsAhead(base, branch string) (int, error) {
 	return count, nil
 }
 
+// CountCommitsBehind returns the number of commits that HEAD is behind the given ref.
+// For example, CountCommitsBehind("origin/main") returns how many commits
+// are on origin/main that are not on the current HEAD.
+func (g *Git) CountCommitsBehind(ref string) (int, error) {
+	out, err := g.run("rev-list", "--count", "HEAD.."+ref)
+	if err != nil {
+		return 0, err
+	}
+
+	var count int
+	_, err = fmt.Sscanf(out, "%d", &count)
+	if err != nil {
+		return 0, fmt.Errorf("parsing commit count: %w", err)
+	}
+
+	return count, nil
+}
+
 // StashCount returns the number of stashes in the repository.
 func (g *Git) StashCount() (int, error) {
 	out, err := g.run("stash", "list")