perf(gateway): stop spawning git for paths that cannot answer

The project tree probes every distinct session cwd, and on a long-lived
history most of those directories are deleted worktrees — `git -C` there
can only fail, at the price of a fork each. Stat first.

The second elision is `common_repo_root`: only repos have a common dir,
and the parallel warm never covers that probe because `resolve()` reaches
it only for cwds that already resolved. Every non-repo cwd was therefore
paying a serial `git` spawn on the discovery pass.

tests/tui_gateway/test_projects_rpc.py

@@ -146,6 +146,38 @@ def test_warm_roots_probes_in_parallel_and_fills_the_cache(monkeypatch):
     assert live["calls"] == before
 
 
+def test_missing_directory_costs_no_subprocess(monkeypatch):
+    # Deleted worktrees dominate a long session history's cwds, and `git -C` on
+    # one can only fail — so it must never reach the fork.
+    from tui_gateway import git_probe
+
+    def boom(*_a, **_kw):
+        raise AssertionError("spawned git for a directory that does not exist")
+
+    monkeypatch.setattr(git_probe, "bounded_git_probe", boom)
+
+    assert git_probe.run_git("/gone/worktree", "rev-parse", "--show-toplevel") == ""
+
+
+def test_non_repo_cwd_is_not_probed_for_a_common_dir(monkeypatch, tmp_path):
+    # `warm_roots` only reaches `common_repo_root` for cwds that ARE repos, so a
+    # common-dir probe here is one the warm can't absorb: it runs serially on
+    # the discovery pass, once per non-repo cwd.
+    from tui_gateway import git_probe
+
+    git_probe.invalidate()
+    asked = []
+
+    def probe(cwd, *args):
+        asked.append(args[-1])
+        return ""  # not a repo, whatever we ask
+
+    monkeypatch.setattr(git_probe, "run_git", probe)
+
+    assert git_probe.common_repo_root(str(tmp_path)) == ""
+    assert asked == ["--show-toplevel"]
+
+
 def test_create_list_roundtrip(tmp_path):
     created = _call("projects.create", {"name": "Demo", "folders": [str(tmp_path)], "use": True})
     assert created["project"]["slug"] == "demo"

tui_gateway/git_probe.py

@@ -50,7 +50,11 @@ def run_git(cwd: str, *args: str) -> str:
     session readiness when a killed git left a suspended descendant holding the
     pipe handles (issue #68609).
     """
-    if not cwd:
+    if not cwd or not os.path.isdir(cwd):
+        # `git -C` on a directory that no longer exists can only fail, and it
+        # fails at the price of a fork. Deleted worktrees dominate the cwds a
+        # long-lived session history hands us, so the stat pays for itself many
+        # times over on every project-tree build.
         return ""
     return bounded_git_probe(["git", "-C", cwd, *args], timeout=_GIT_TIMEOUT)
 
@@ -153,6 +157,13 @@ def common_repo_root(cwd: str) -> str:
     if not cwd:
         return ""
 
+    # No work tree, nothing to fold. Reading the (warmed, negative-cached)
+    # toplevel first spares every non-repo cwd a second `git` spawn — one the
+    # parallel warm can never absorb, since `resolve()` only reaches here for
+    # cwds that ARE repos.
+    if not repo_root(cwd):
+        return ""
+
     def _probe() -> str:
         gitdir = run_git(cwd, "rev-parse", "--path-format=absolute", "--git-common-dir")
         if gitdir: