fix(state): restrict sensitive store file permissions

response_store.db (api server) holds conversation history including tool
payloads, prompts, and results. webhook_subscriptions.json holds per-route
HMAC secrets. Under a permissive umask (e.g. 0o022, default on most
distros) both files were created mode 0o644 — readable by other local
users on shared boxes.

- gateway/platforms/api_server.py: ResponseStore tightens itself + WAL/SHM
  sidecars to 0o600 after __init__, then trusts the inode. (Original
  contributor patch chmod'd after every _commit() — wasteful on a hot
  api_server path; chmod-on-create is sufficient since SQLite preserves
  mode bits across writes.)

- hermes_cli/webhook.py: _save_subscriptions writes via tempfile.mkstemp
  (which itself creates the file with 0o600), chmods the temp before the
  atomic rename, and re-asserts 0o600 on the destination so an existing
  permissive file from before this fix gets narrowed.

Tests cover (a) creation under permissive umask leaves 0o600 and (b) an
existing 0o644 webhook_subscriptions.json gets narrowed on next save.
Tests guarded with skipif os.name=='nt' since POSIX mode bits don't apply
on Windows.

Salvaged from PR #30917 by @Hinotoi-agent. Reworked the api_server.py
side from chmod-on-every-commit to chmod-on-create.

Co-authored-by: teknium1 <127238744+teknium1@users.noreply.github.com>

hermes_cli/webhook.py

@@ -11,8 +11,10 @@ hot-reloaded by the webhook adapter without a gateway restart.
 """
 
 import json
+import os
 import re
 import secrets
+import tempfile
 import time
 from pathlib import Path
 from typing import Dict
@@ -23,6 +25,7 @@ from hermes_cli.config import cfg_get
 
 
 _SUBSCRIPTIONS_FILENAME = "webhook_subscriptions.json"
+_SUBSCRIPTIONS_FILE_MODE = 0o600
 
 
 def _hermes_home() -> Path:
@@ -48,12 +51,33 @@ def _load_subscriptions() -> Dict[str, dict]:
 def _save_subscriptions(subs: Dict[str, dict]) -> None:
     path = _subscriptions_path()
     path.parent.mkdir(parents=True, exist_ok=True)
-    tmp_path = path.with_suffix(".tmp")
-    tmp_path.write_text(
-        json.dumps(subs, indent=2, ensure_ascii=False),
-        encoding="utf-8",
+    # webhook_subscriptions.json contains per-route HMAC secrets — write
+    # via tempfile + chmod 0o600 before the atomic rename so a permissive
+    # umask cannot leave the secrets readable to other local users in the
+    # window between create and rename.
+    fd, tmp_name = tempfile.mkstemp(
+        prefix=f".{path.name}.",
+        suffix=".tmp",
+        dir=path.parent,
+        text=True,
     )
-    atomic_replace(tmp_path, path)
+    tmp_path = Path(tmp_name)
+    try:
+        with os.fdopen(fd, "w", encoding="utf-8") as fh:
+            json.dump(subs, fh, indent=2, ensure_ascii=False)
+            fh.flush()
+            os.fsync(fh.fileno())
+        os.chmod(tmp_path, _SUBSCRIPTIONS_FILE_MODE)
+        atomic_replace(tmp_path, path)
+        # Re-assert after rename in case the destination existed with a
+        # broader mode and atomic_replace preserved it.
+        os.chmod(path, _SUBSCRIPTIONS_FILE_MODE)
+    except Exception:
+        try:
+            tmp_path.unlink(missing_ok=True)
+        except OSError:
+            pass
+        raise
 
 
 def _get_webhook_config() -> dict: