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/migrate_sync.go

@@ -148,7 +148,12 @@ func runMigrateSync(ctx context.Context, branchName string, dryRun, force bool)
 			fmt.Printf("→ Would create new branch '%s'\n", branchName)
 		}
 
-		worktreePath := filepath.Join(repoRoot, ".git", "beads-worktrees", branchName)
+		// Use worktree-aware git directory detection
+		gitDir, err := git.GetGitDir()
+		if err != nil {
+			return fmt.Errorf("not a git repository: %w", err)
+		}
+		worktreePath := filepath.Join(gitDir, "beads-worktrees", branchName)
 		fmt.Printf("→ Would create worktree at: %s\n", worktreePath)
 
 		fmt.Println("\n=== END DRY RUN ===")
@@ -183,7 +188,12 @@ func runMigrateSync(ctx context.Context, branchName string, dryRun, force bool)
 	}
 
 	// Step 2: Create the worktree
-	worktreePath := filepath.Join(repoRoot, ".git", "beads-worktrees", branchName)
+	// Use worktree-aware git directory detection
+	gitDir, err := git.GetGitDir()
+	if err != nil {
+		return fmt.Errorf("not a git repository: %w", err)
+	}
+	worktreePath := filepath.Join(gitDir, "beads-worktrees", branchName)
 	fmt.Printf("→ Creating worktree at %s...\n", worktreePath)
 
 	wtMgr := git.NewWorktreeManager(repoRoot)