fix(data): merge preflights ignore untracked files in target working tree

HasChangesAsync counted untracked files, so a stray file in the shared
target working dir (e.g. left by a concurrent session) blocked merge
preflights even though nothing tracked changed. Add an includeUntracked
overload defaulting to true, and pass includeUntracked: false only from
the two target-working-tree merge preflights (TaskMergeService.MergeAsync,
PlanningMergeOrchestrator.StartAsync). Auto-commit and the worktree
cleanup data-loss guard keep counting untracked files, since those
callers need to know about them.

src/ClaudeDo.Data/Git/GitService.cs

@@ -71,9 +71,22 @@ public sealed class GitService
         return stdout;
     }
 
-    public async Task<bool> HasChangesAsync(string worktreePath, CancellationToken ct = default)
+    public Task<bool> HasChangesAsync(string worktreePath, CancellationToken ct = default) =>
+        HasChangesAsync(worktreePath, includeUntracked: true, ct);
+
+    /// <summary>
+    /// Uncommitted-changes check. <paramref name="includeUntracked"/>=false ignores untracked
+    /// files — use this for merge preflights on a shared target working dir, where stray
+    /// untracked files (e.g. from a concurrent session) shouldn't block a merge. Auto-commit
+    /// and data-loss-guard callers keep the default (true): a new file a task created, or an
+    /// untracked file about to be discarded, is a real uncommitted change.
+    /// </summary>
+    public async Task<bool> HasChangesAsync(string worktreePath, bool includeUntracked, CancellationToken ct = default)
     {
-        var (exitCode, stdout, stderr) = await RunGitAsync(worktreePath, ["status", "--porcelain"], ct);
+        string[] args = includeUntracked
+            ? ["status", "--porcelain"]
+            : ["status", "--porcelain", "--untracked-files=no"];
+        var (exitCode, stdout, stderr) = await RunGitAsync(worktreePath, args, ct);
         if (exitCode != 0)
             throw new InvalidOperationException($"git status --porcelain failed (exit {exitCode}): {stderr}");
         return !string.IsNullOrWhiteSpace(stdout);