fix(hooks): use Optional[Path] for py39 compat

PEP 604 union syntax (Path | None) requires Python 3.10+. The project
still supports 3.9 (per pyproject target-version and CI matrix), and
this annotation lives in a function signature so it is evaluated at
module load time — failing with "unsupported operand type(s) for |"
on test-linux 3.9.

The other ``int | None`` annotation in this file is inside a function
body, where Python skips runtime evaluation of local annotations, so
it does not trip 3.9.

mempalace/hooks_cli.py

@@ -14,6 +14,7 @@ import subprocess
 import sys
 from datetime import datetime
 from pathlib import Path
+from typing import Optional
 
 SAVE_INTERVAL = 15
 STATE_DIR = Path.home() / ".mempalace" / "hook_state"
@@ -343,7 +344,7 @@ def _mine_already_running(cmd: list[str]) -> bool:
     return _pid_alive(int(recorded))
 
 
-def _claim_mine_slot(cmd: list[str]) -> Path | None:
+def _claim_mine_slot(cmd: list[str]) -> Optional[Path]:
     """Atomically reserve the per-target PID slot for ``cmd``.
 
     Returns the slot path on success, or ``None`` if the target is