fix(polecat): ignore .beads/ files when detecting uncommitted work

Add CleanExcludingBeads() method that returns true if the only uncommitted
changes are .beads/ database files. These files are synced across worktrees
and shouldn't block polecat cleanup.

Fixes #516

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

internal/polecat/manager.go

@@ -1021,9 +1021,9 @@ func (m *Manager) DetectStalePolecats(threshold int) ([]*StalenessInfo, error) {
 		polecatGit := git.NewGit(p.ClonePath)
 		info.CommitsBehind = countCommitsBehind(polecatGit, defaultBranch)
 
-		// Check for uncommitted work
+		// Check for uncommitted work (excluding .beads/ files which are synced across worktrees)
 		status, err := polecatGit.CheckUncommittedWork()
-		if err == nil && !status.Clean() {
+		if err == nil && !status.CleanExcludingBeads() {
 			info.HasUncommittedWork = true
 		}