Fix git hooks not working in worktrees (#1126)

Git hooks are shared across all worktrees and live in the common git
directory (e.g., /repo/.git/hooks), not the worktree-specific directory
(e.g., /repo/.git/worktrees/feature/hooks).

The core issue was in GetGitHooksDir() which used GetGitDir() instead
of GetGitCommonDir(). This caused hooks to be installed to/read from
the wrong location when running in a worktree.

Additionally, several places in the codebase manually constructed
hooks paths using gitDir + "hooks" instead of calling GetGitHooksDir().
These have been updated to use the proper worktree-aware path.

Affected areas:
- GetGitHooksDir() now uses GetGitCommonDir()
- CheckGitHooks() uses GetGitHooksDir()
- installHooks/uninstallHooks use GetGitHooksDir()
- runChainedHook() uses GetGitHooksDir()
- Doctor checks use git-common-dir for hooks paths
- Reset command uses GetGitCommonDir() for hooks and beads-worktrees

Symptoms that this fixes:
- Chained hooks (pre-commit.old) not running in worktrees
- bd hooks install not finding/installing hooks correctly in worktrees
- bd hooks list showing incorrect status in worktrees
- bd doctor reporting incorrect hooks status in worktrees

Co-authored-by: Zain Rizvi <4468967+ZainRizvi@users.noreply.github.com>

cmd/bd/doctor/fix/hooks.go

@@ -108,20 +108,20 @@ var hookManagerPatterns = []hookManagerPattern{
 // DetectActiveHookManager reads the git hooks to determine which manager installed them.
 // This is more reliable than just checking for config files when multiple managers exist.
 func DetectActiveHookManager(path string) string {
-	// Get git dir
-	cmd := exec.Command("git", "rev-parse", "--git-dir")
+	// Get common git dir (hooks are shared across worktrees)
+	cmd := exec.Command("git", "rev-parse", "--git-common-dir")
 	cmd.Dir = path
 	output, err := cmd.Output()
 	if err != nil {
 		return ""
 	}
-	gitDir := strings.TrimSpace(string(output))
-	if !filepath.IsAbs(gitDir) {
-		gitDir = filepath.Join(path, gitDir)
+	gitCommonDir := strings.TrimSpace(string(output))
+	if !filepath.IsAbs(gitCommonDir) {
+		gitCommonDir = filepath.Join(path, gitCommonDir)
 	}
 
 	// Check for custom hooks path (core.hooksPath)
-	hooksDir := filepath.Join(gitDir, "hooks")
+	hooksDir := filepath.Join(gitCommonDir, "hooks")
 	hooksPathCmd := exec.Command("git", "config", "--get", "core.hooksPath")
 	hooksPathCmd.Dir = path
 	if hooksPathOutput, err := hooksPathCmd.Output(); err == nil {