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/hooks.go

@@ -11,20 +11,9 @@ import (
 	"strings"
 
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/beads/internal/git"
 )
 
-// getGitDir returns the actual .git directory path.
-// In a normal repo, this is ".git". In a worktree, .git is a file
-// containing "gitdir: /path/to/actual/git/dir", so we use git rev-parse.
-func getGitDir() (string, error) {
-	cmd := exec.Command("git", "rev-parse", "--git-dir")
-	output, err := cmd.Output()
-	if err != nil {
-		return "", fmt.Errorf("not a git repository: %w", err)
-	}
-	return strings.TrimSpace(string(output)), nil
-}
-
 //go:embed templates/hooks/*
 var hooksFS embed.FS
 
@@ -59,7 +48,7 @@ func CheckGitHooks() []HookStatus {
 	statuses := make([]HookStatus, 0, len(hooks))
 
 	// Get actual git directory (handles worktrees)
-	gitDir, err := getGitDir()
+	gitDir, err := git.GetGitDir()
 	if err != nil {
 		// Not a git repo - return all hooks as not installed
 		for _, hookName := range hooks {
@@ -299,7 +288,7 @@ var hooksListCmd = &cobra.Command{
 
 func installHooks(embeddedHooks map[string]string, force bool, shared bool) error {
 	// Get actual git directory (handles worktrees where .git is a file)
-	gitDir, err := getGitDir()
+	gitDir, err := git.GetGitDir()
 	if err != nil {
 		return err
 	}
@@ -361,7 +350,7 @@ func configureSharedHooksPath() error {
 
 func uninstallHooks() error {
 	// Get actual git directory (handles worktrees)
-	gitDir, err := getGitDir()
+	gitDir, err := git.GetGitDir()
 	if err != nil {
 		return err
 	}