fix: support git worktrees in hooks installation

Use `git rev-parse --git-dir` instead of hardcoded `.git` path to find
the actual git directory. In worktrees, `.git` is a file containing a
gitdir pointer, not a directory.

Changes:
- Add getGitDir() helper in hooks.go
- Update installHooks(), uninstallHooks(), CheckGitHooks() to use it
- Update hooksInstalled(), detectExistingHooks(), installGitHooks() in init.go
- Update checkHooksQuick() in doctor.go
- Update GitHooks() in doctor/fix/hooks.go
- Update tests to use real git repos via `git init`

Fixes bd-63l

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/init.go

@@ -459,8 +459,12 @@ func init() {
 
 // hooksInstalled checks if bd git hooks are installed
 func hooksInstalled() bool {
-	preCommit := filepath.Join(".git", "hooks", "pre-commit")
-	postMerge := filepath.Join(".git", "hooks", "post-merge")
+	gitDir, err := getGitDir()
+	if err != nil {
+		return false
+	}
+	preCommit := filepath.Join(gitDir, "hooks", "pre-commit")
+	postMerge := filepath.Join(gitDir, "hooks", "post-merge")
 
 	// Check if both hooks exist
 	_, err1 := os.Stat(preCommit)
@@ -515,7 +519,11 @@ type hookInfo struct {
 
 // detectExistingHooks scans for existing git hooks
 func detectExistingHooks() []hookInfo {
-	hooksDir := filepath.Join(".git", "hooks")
+	gitDir, err := getGitDir()
+	if err != nil {
+		return nil
+	}
+	hooksDir := filepath.Join(gitDir, "hooks")
 	hooks := []hookInfo{
 		{name: "pre-commit", path: filepath.Join(hooksDir, "pre-commit")},
 		{name: "post-merge", path: filepath.Join(hooksDir, "post-merge")},
@@ -569,7 +577,11 @@ func promptHookAction(existingHooks []hookInfo) string {
 
 // installGitHooks installs git hooks inline (no external dependencies)
 func installGitHooks() error {
-	hooksDir := filepath.Join(".git", "hooks")
+	gitDir, err := getGitDir()
+	if err != nil {
+		return err
+	}
+	hooksDir := filepath.Join(gitDir, "hooks")
 
 	// Ensure hooks directory exists
 	if err := os.MkdirAll(hooksDir, 0750); err != nil {