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

internal/storage/sqlite/queries.go

@@ -1194,7 +1194,7 @@ func (s *SQLiteStorage) executeDelete(ctx context.Context, tx *sql.Tx, inClause
 	for rows.Next() {
 		var id, issueType string
 		if err := rows.Scan(&id, &issueType); err != nil {
-			_ = rows.Close()
+			_ = rows.Close() // #nosec G104 - error handling not critical in error path
 			return fmt.Errorf("failed to scan issue type: %w", err)
 		}
 		issueTypes[id] = issueType