fix(hooks): detach Popen children so the hook can exit on Windows

The Stop hook spawns mining subprocesses via subprocess.Popen and then
returns. On Windows the parent stays blocked at session end because the
child inherits stdout/stderr handles and the OS waits for them to
release before the parent can exit — the user-visible symptom is the
"running stop hooks... 3/3" spinner hanging for minutes (#1268).

Add _detached_popen_kwargs() helper that returns the right detach knobs
per platform:
- POSIX: start_new_session=True, stdin=DEVNULL, close_fds=True
- Windows: creationflags=DETACHED_PROCESS|CREATE_NEW_PROCESS_GROUP|
  CREATE_BREAKAWAY_FROM_JOB, stdin=DEVNULL, close_fds=True

Apply to all three fire-and-forget Popen sites in hooks_cli:
_spawn_mine, _ingest_transcript, _desktop_toast. Leave _mine_sync's
subprocess.run alone — that path is intentionally synchronous (the
precompact hook must wait for the mine to finish).

Note: the issue body references mempalace-stop.js, which does not exist
in this repo (the plugin ships shell wrappers calling Python). The
mechanism described — child holds parent open via inherited handles —
is universal, so this fix targets the equivalent symptom in our Python
hook path. Will follow up on the upstream JS file with the reporter.

mempalace/hooks_cli.py

@@ -19,6 +19,27 @@ STATE_DIR = Path.home() / ".mempalace" / "hook_state"
 PALACE_ROOT = Path.home() / ".mempalace"
 
 
+def _detached_popen_kwargs() -> dict:
+    """Kwargs that fully detach a Popen child so the hook process can exit.
+
+    Without these, Windows holds the parent open until the child closes the
+    inherited stdout/stderr handles — manifesting as "Stop hook hangs" at
+    session end (#1268). On POSIX the parent can already exit (orphan
+    reparents to init), but ``start_new_session`` makes the boundary
+    explicit so signals to the hook don't propagate to the background mine.
+    """
+    kwargs: dict = {"stdin": subprocess.DEVNULL, "close_fds": True}
+    if os.name == "nt":
+        flags = 0
+        for name in ("DETACHED_PROCESS", "CREATE_NEW_PROCESS_GROUP", "CREATE_BREAKAWAY_FROM_JOB"):
+            flags |= getattr(subprocess, name, 0)
+        if flags:
+            kwargs["creationflags"] = flags
+    else:
+        kwargs["start_new_session"] = True
+    return kwargs
+
+
 def _palace_root_exists() -> bool:
     """User-removable kill-switch.
 
@@ -285,7 +306,7 @@ def _spawn_mine(cmd: list) -> None:
     STATE_DIR.mkdir(parents=True, exist_ok=True)
     log_path = STATE_DIR / "hook.log"
     with open(log_path, "a") as log_f:
-        proc = subprocess.Popen(cmd, stdout=log_f, stderr=log_f)
+        proc = subprocess.Popen(cmd, stdout=log_f, stderr=log_f, **_detached_popen_kwargs())
     _MINE_PID_FILE.write_text(str(proc.pid))
 
 
@@ -350,6 +371,7 @@ def _desktop_toast(body: str, title: str = "MemPalace"):
             ["notify-send", "--app-name=MemPalace", "--icon=brain", title, body],
             stdout=subprocess.DEVNULL,
             stderr=subprocess.DEVNULL,
+            **_detached_popen_kwargs(),
         )
     except OSError:
         pass
@@ -513,6 +535,7 @@ def _ingest_transcript(transcript_path: str):
                 ],
                 stdout=log_f,
                 stderr=log_f,
+                **_detached_popen_kwargs(),
             )
         _log(f"Transcript ingest started: {path.name}")
     except OSError: