fix: bd sync fails with exit 128 in bare repo worktrees (GH#827)

Two fixes for bare repo worktree setups:

1. fork_protection.go: Use git.GetGitDir() instead of hardcoding .git
   In worktrees, .git is a file containing gitdir path, not a directory.
   Using GetGitDir() handles this correctly.

2. sync_git.go: Simplify gitHasBeadsChanges to use absolute paths
   The previous code used git -C main-repo-root status, but in bare
   repo worktrees GetMainRepoRoot() returns the parent of the bare repo
   which is not a valid working tree. Using absolute paths without -C
   lets git find the repo from cwd, which always works.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/fork_protection.go

@@ -43,8 +43,15 @@ func ensureForkProtection() {
 		return // Not a fork of beads, user's own project
 	}
 
+	// Get actual git directory (handles worktrees where .git is a file) (GH#827)
+	gitDir, err := git.GetGitDir()
+	if err != nil {
+		debug.Printf("fork protection: failed to get git dir: %v", err)
+		return
+	}
+
 	// Check if already excluded
-	excludePath := filepath.Join(gitRoot, ".git", "info", "exclude")
+	excludePath := filepath.Join(gitDir, "info", "exclude")
 	if isAlreadyExcluded(excludePath) {
 		return
 	}