feat(doctor): detect and fix stale sync branches (bd-6rf)

Add 'Sync Branch Health' check that detects:
1. Local sync branch diverged from remote (after force-push reset)
2. Sync branch significantly behind main on source files (20+ commits, 50+ files)

Add --fix support that:
- Handles worktree case (resets within worktree)
- Handles regular branch case (deletes and recreates from main)
- Pushes the reset branch to remote

This helps contributors whose local beads-sync becomes orphaned after
someone else resets the branch.

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

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

cmd/bd/doctor/fix/sync_branch.go

@@ -42,3 +42,111 @@ func SyncBranchConfig(path string) error {
 	fmt.Printf("  Set sync.branch = %s\n", currentBranch)
 	return nil
 }
+
+// SyncBranchHealth fixes a stale or diverged sync branch by resetting it to main.
+// This handles two cases:
+// 1. Local sync branch diverged from remote (after force-push)
+// 2. Sync branch far behind main on source files
+//
+// bd-6rf: Detect and fix stale beads-sync branch
+func SyncBranchHealth(path, syncBranch string) error {
+	if err := validateBeadsWorkspace(path); err != nil {
+		return err
+	}
+
+	// Determine main branch
+	mainBranch := "main"
+	cmd := exec.Command("git", "rev-parse", "--verify", "main")
+	cmd.Dir = path
+	if err := cmd.Run(); err != nil {
+		cmd = exec.Command("git", "rev-parse", "--verify", "master")
+		cmd.Dir = path
+		if err := cmd.Run(); err != nil {
+			return fmt.Errorf("cannot determine main branch (neither main nor master exists)")
+		}
+		mainBranch = "master"
+	}
+
+	// Check if there's a worktree for this branch
+	worktreePath := ""
+	cmd = exec.Command("git", "worktree", "list", "--porcelain")
+	cmd.Dir = path
+	output, err := cmd.Output()
+	if err == nil {
+		lines := strings.Split(string(output), "\n")
+		for i, line := range lines {
+			if strings.HasPrefix(line, "worktree ") {
+				wt := strings.TrimPrefix(line, "worktree ")
+				// Check if next line has the branch
+				if i+2 < len(lines) && strings.Contains(lines[i+2], syncBranch) {
+					worktreePath = wt
+					break
+				}
+			}
+		}
+	}
+
+	// If worktree exists, reset within it
+	if worktreePath != "" {
+		fmt.Printf("  Resetting sync branch in worktree: %s\n", worktreePath)
+		cmd = exec.Command("git", "fetch", "origin", mainBranch)
+		cmd.Dir = worktreePath
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return fmt.Errorf("failed to fetch: %w\n%s", err, out)
+		}
+
+		cmd = exec.Command("git", "reset", "--hard", fmt.Sprintf("origin/%s", mainBranch))
+		cmd.Dir = worktreePath
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return fmt.Errorf("failed to reset worktree: %w\n%s", err, out)
+		}
+
+		// Push the reset branch
+		cmd = exec.Command("git", "push", "--force-with-lease", "origin", syncBranch)
+		cmd.Dir = worktreePath
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return fmt.Errorf("failed to push: %w\n%s", err, out)
+		}
+
+		fmt.Printf("  ✓ Reset %s to %s and pushed\n", syncBranch, mainBranch)
+		return nil
+	}
+
+	// No worktree - reset the branch directly
+	// First, make sure we're not on the sync branch
+	cmd = exec.Command("git", "symbolic-ref", "--short", "HEAD")
+	cmd.Dir = path
+	currentBranchOutput, err := cmd.Output()
+	if err == nil && strings.TrimSpace(string(currentBranchOutput)) == syncBranch {
+		return fmt.Errorf("currently on %s branch - checkout a different branch first", syncBranch)
+	}
+
+	// Delete and recreate the branch from main
+	fmt.Printf("  Deleting local %s branch...\n", syncBranch)
+	cmd = exec.Command("git", "branch", "-D", syncBranch)
+	cmd.Dir = path
+	_ = cmd.Run() // Ignore error if branch doesn't exist
+
+	// Fetch latest and recreate
+	cmd = exec.Command("git", "fetch", "origin", mainBranch)
+	cmd.Dir = path
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("failed to fetch: %w\n%s", err, out)
+	}
+
+	cmd = exec.Command("git", "branch", syncBranch, fmt.Sprintf("origin/%s", mainBranch))
+	cmd.Dir = path
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("failed to create branch: %w\n%s", err, out)
+	}
+
+	// Push the new branch
+	cmd = exec.Command("git", "push", "--force-with-lease", "origin", syncBranch)
+	cmd.Dir = path
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("failed to push: %w\n%s", err, out)
+	}
+
+	fmt.Printf("  ✓ Recreated %s from %s and pushed\n", syncBranch, mainBranch)
+	return nil
+}