Fix rebuilds from deleted hook cwd

What changed
- Stabilize relative rebuild execution before graphify-out queue/lock setup.
- Recover from deleted transient hook CWDs when GRAPHIFY_REPO_ROOT points at the repository root.
- Fail cleanly when the current working directory is gone and no repo root fallback is available.
- Add regression coverage for both fallback and clean-failure paths.

Why
- Detached post-commit/post-checkout rebuilds can inherit a transient CWD that is deleted before the background process starts.
- _rebuild_code previously used relative graphify-out paths before Path.cwd()/watch_path resolution could be handled, causing FileNotFoundError: [Errno 2] No such file or directory.

Validation
- .venv/bin/pytest tests/test_watch.py -q: 54 passed, 2 skipped.
- .venv/bin/ruff check graphify/watch.py tests/test_watch.py: passed.
- .venv/bin/python -m py_compile graphify/watch.py tests/test_watch.py: passed.
- env -u PYTHONPATH -u PYTHONHOME PYTHONHASHSEED=0 .venv/bin/pytest -q: 2904 passed, 30 skipped.

Notes
- Full suite without PYTHONHASHSEED=0 exposed an unrelated ordering-sensitive labeling test; with the deterministic hash seed used by graphify hooks, the suite passes.

graphify/watch.py

@@ -657,6 +657,37 @@ def _json_text(data: dict) -> str:
     return json.dumps(data, indent=2, ensure_ascii=False) + "\n"
 
 
+def _stabilize_rebuild_cwd(watch_path: Path) -> bool:
+    """Ensure relative rebuild paths have a usable CWD before queue/lock setup.
+
+    Detached git hooks can inherit a transient working directory that is deleted
+    before the background rebuild starts. In that state Path.cwd(),
+    Path('.').resolve(), and relative graphify-out mkdirs raise FileNotFoundError
+    before the normal rebuild error handling can run. Hooks that know the repo
+    root export GRAPHIFY_REPO_ROOT so the rebuild can recover by chdir'ing there.
+    """
+    if watch_path.is_absolute():
+        return True
+
+    repo_root = os.environ.get("GRAPHIFY_REPO_ROOT", "").strip()
+    if repo_root and Path(repo_root).is_dir():
+        try:
+            os.chdir(repo_root)
+            return True
+        except OSError:
+            pass
+
+    try:
+        Path.cwd()
+        return True
+    except FileNotFoundError:
+        print(
+            "[graphify watch] Rebuild failed: current working directory "
+            "no longer exists and GRAPHIFY_REPO_ROOT is not set."
+        )
+        return False
+
+
 def _rebuild_code(
     watch_path: Path,
     *,
@@ -690,6 +721,9 @@ def _rebuild_code(
 
     Returns True on success, False on error or skipped-due-to-lock.
     """
+    if not _stabilize_rebuild_cwd(watch_path):
+        return False
+
     out = watch_path / _GRAPHIFY_OUT
     if acquire_lock:
         # #1059: incremental (changed_paths is not None) hooks must not drop

tests/test_watch.py

@@ -176,6 +176,56 @@ def test_graphify_root_preserves_absolute_when_user_supplied(tmp_path):
     )
 
 
+def test_rebuild_code_deleted_cwd_without_repo_root_returns_false(tmp_path, monkeypatch, capsys):
+    """Detached hooks can inherit a CWD that no longer exists.
+
+    Without GRAPHIFY_REPO_ROOT, the rebuild should fail cleanly before creating
+    relative graphify-out queue/lock files.
+    """
+    from graphify.watch import _rebuild_code
+
+    old_cwd = Path.cwd()
+    gone = tmp_path / "gone"
+    gone.mkdir()
+    monkeypatch.delenv("GRAPHIFY_REPO_ROOT", raising=False)
+
+    os.chdir(gone)
+    gone.rmdir()
+    try:
+        assert _rebuild_code(Path("."), changed_paths=[Path("lib.py")]) is False
+    finally:
+        os.chdir(old_cwd)
+
+    out = capsys.readouterr().out
+    assert "current working directory no longer exists" in out
+
+
+def test_rebuild_code_deleted_cwd_uses_graphify_repo_root(tmp_path, monkeypatch):
+    """GRAPHIFY_REPO_ROOT lets detached hook rebuilds recover from a deleted CWD."""
+    from graphify.watch import _rebuild_code
+
+    old_cwd = Path.cwd()
+    corpus = tmp_path / "corpus"
+    corpus.mkdir()
+    (corpus / "lib.py").write_text("def f(): pass\n", encoding="utf-8")
+    gone = tmp_path / "gone"
+    gone.mkdir()
+    monkeypatch.setenv("GRAPHIFY_REPO_ROOT", str(corpus))
+
+    os.chdir(gone)
+    gone.rmdir()
+    try:
+        assert _rebuild_code(
+            Path("."),
+            changed_paths=[Path("lib.py")],
+            no_cluster=True,
+        ) is True
+        assert Path.cwd().resolve() == corpus.resolve()
+        assert (corpus / "graphify-out" / "graph.json").exists()
+    finally:
+        os.chdir(old_cwd)
+
+
 def test_rebuild_code_evicts_nodes_from_deleted_files(tmp_path):
     """#1007: graphify update (_rebuild_code with no changed_paths) must remove
     nodes and edges from files deleted since the last run."""