feat: add Git worktree compatibility (PR #478)

Adds comprehensive Git worktree support for beads issue tracking:

Core changes:
- New internal/git/gitdir.go package for worktree detection
- GetGitDir() returns proper .git location (main repo, not worktree)
- Updated all hooks to use git.GetGitDir() instead of local helper
- BeadsDir() now prioritizes main repository's .beads directory

Features:
- Hooks auto-install in main repo when run from worktree
- Shared .beads directory across all worktrees
- Config option no-install-hooks to disable auto-install
- New bd worktree subcommand for diagnostics

Documentation:
- New docs/WORKTREES.md with setup instructions
- Updated CHANGELOG.md and AGENT_INSTRUCTIONS.md

Testing:
- Updated tests to use exported git.GetGitDir()
- Added worktree detection tests

Co-authored-by: Claude <noreply@anthropic.com>
Closes: #478

cmd/bd/compact.go

@@ -1019,7 +1019,7 @@ func pruneExpiredTombstones() (*TombstonePruneResult, error) {
 		}
 		allIssues = append(allIssues, &issue)
 	}
-	_ = file.Close() // Best effort close, already read all data
+	file.Close()
 
 	// Determine TTL
 	ttl := types.DefaultTombstoneTTL
@@ -1052,20 +1052,20 @@ func pruneExpiredTombstones() (*TombstonePruneResult, error) {
 	encoder := json.NewEncoder(tempFile)
 	for _, issue := range kept {
 		if err := encoder.Encode(issue); err != nil {
-			_ = tempFile.Close()
-			_ = os.Remove(tempPath) // Best effort cleanup
+			tempFile.Close()
+			os.Remove(tempPath)
 			return nil, fmt.Errorf("failed to write issue %s: %w", issue.ID, err)
 		}
 	}
 
 	if err := tempFile.Close(); err != nil {
-		_ = os.Remove(tempPath) // Best effort cleanup
+		os.Remove(tempPath)
 		return nil, fmt.Errorf("failed to close temp file: %w", err)
 	}
 
 	// Atomically replace
 	if err := os.Rename(tempPath, issuesPath); err != nil {
-		_ = os.Remove(tempPath) // Best effort cleanup
+		os.Remove(tempPath)
 		return nil, fmt.Errorf("failed to replace issues.jsonl: %w", err)
 	}