fix(hooks): use is_dir() for palace root check (review feedback)

Both @igorls and the Qodo bot flagged that `_palace_root_exists()` used
`Path.exists()`, which returns True for a regular file. A stray file at
`~/.mempalace` would let the kill-switch be bypassed and crash later in
`STATE_DIR.mkdir()` with NotADirectoryError.

Switched to `Path.is_dir()`. Also fold `_log()`'s inline check through
`_palace_root_exists()` so both kill-switch sites use the same predicate.

New test pins the behavior: a regular file at the palace root path is
treated as absent (hook short-circuits, _log does not crash, the stray
file is left untouched).

tests/test_hooks_cli.py

@@ -1035,3 +1035,35 @@ def test_existing_dir_proceeds_normally(tmp_path, monkeypatch):
     # _log should have created the state dir under the existing palace root
     assert (fake_root / "hook_state").exists()
     assert (fake_root / "hook_state" / "hook.log").is_file()
+
+
+def test_regular_file_at_palace_root_treated_as_absent(tmp_path, monkeypatch):
+    """A regular file at ~/.mempalace must be treated the same as absent.
+
+    ``Path.exists()`` returns True for a regular file, which would let the
+    kill-switch be bypassed and crash later when ``STATE_DIR.mkdir()`` runs
+    on ``NotADirectoryError``. ``_palace_root_exists()`` must use
+    ``is_dir()`` so a stray file (or broken symlink) short-circuits cleanly.
+    """
+    fake_root = tmp_path / "file-not-dir"
+    fake_root.write_text("oops, this is a file not a directory")
+    monkeypatch.setattr(hooks_cli_mod, "PALACE_ROOT", fake_root)
+    monkeypatch.setattr(hooks_cli_mod, "STATE_DIR", fake_root / "hook_state")
+    monkeypatch.setattr(hooks_cli_mod, "_state_dir_initialized", False)
+
+    # _palace_root_exists() is the source of truth — it must return False.
+    assert hooks_cli_mod._palace_root_exists() is False
+
+    # Hooks must short-circuit (return {} on stdout) and not touch disk.
+    buf = io.StringIO()
+    with contextlib.redirect_stdout(buf):
+        hook_session_start({"session_id": "file-at-root"}, "claude-code")
+    assert json.loads(buf.getvalue() or "{}") == {}
+
+    # _log must also short-circuit — it must NOT try to mkdir a path under a
+    # regular file (which would raise NotADirectoryError).
+    _log("test message")  # would raise if not short-circuited
+
+    # The stray file is left untouched; we never try to convert it.
+    assert fake_root.is_file()
+    assert fake_root.read_text() == "oops, this is a file not a directory"