chore(claude-do): feat(worktrees): warn about git worktrees ClaudeDo doesn't k

From `docs/open.md` (re-verified against the code 2026-09-02, still open): nothing reconciles `git worktree list` against the `worktrees` table. `ExternalMcpService.ListWorktrees` and the Worktrees overview read DB rows only; `WorktreeManager` calls git solely inside its self-heal path (`ListWorktreePathsForBranchAsync`). Observed once: 22 git-registered worktrees + 26 `claudedo/*` branches in `C:

ClaudeDo-Task: 765fed13-db66-44e4-9285-c4b2d415b443

src/ClaudeDo.Data/Git/GitService.cs

@@ -338,6 +338,38 @@ public sealed class GitService
         return paths;
     }
 
+    /// <summary>All git-registered worktrees for <paramref name="repoDir"/>, with their branch (short name, null if detached).</summary>
+    public async Task<List<(string Path, string? Branch)>> ListWorktreesAsync(string repoDir, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, _) = await RunGitAsync(repoDir, ["worktree", "list", "--porcelain"], ct);
+        if (exitCode != 0) return new();
+
+        var result = new List<(string Path, string? Branch)>();
+        string? currentPath = null;
+        string? currentBranch = null;
+        foreach (var raw in stdout.Split('\n'))
+        {
+            var line = raw.TrimEnd('\r');
+            if (line.StartsWith("worktree ", StringComparison.Ordinal))
+            {
+                currentPath = line["worktree ".Length..].Trim();
+            }
+            else if (line.StartsWith("branch ", StringComparison.Ordinal))
+            {
+                var b = line["branch ".Length..].Trim();
+                currentBranch = b.StartsWith("refs/heads/", StringComparison.Ordinal) ? b["refs/heads/".Length..] : b;
+            }
+            else if (string.IsNullOrWhiteSpace(line))
+            {
+                if (currentPath is not null) result.Add((currentPath, currentBranch));
+                currentPath = null;
+                currentBranch = null;
+            }
+        }
+        if (currentPath is not null) result.Add((currentPath, currentBranch));
+        return result;
+    }
+
     public async Task WorktreePruneAsync(string repoDir, CancellationToken ct = default)
     {
         var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["worktree", "prune"], ct);