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>

tests/hermes_cli/test_webhook_cli.py

@@ -3,6 +3,7 @@
 import json
 import os
 import pytest
+import stat
 from argparse import Namespace
 from pathlib import Path
 
@@ -145,6 +146,31 @@ class TestPersistence:
         path.write_text("broken{{{")
         assert _load_subscriptions() == {}
 
+    @pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits are platform-specific")
+    def test_save_creates_secret_file_owner_only_under_permissive_umask(self):
+        old_umask = os.umask(0o022)
+        try:
+            _save_subscriptions({"demo": {"secret": "TOPSECRET", "prompt": "x"}})
+        finally:
+            os.umask(old_umask)
+
+        path = _subscriptions_path()
+        assert stat.S_IMODE(path.stat().st_mode) == 0o600
+        assert "TOPSECRET" in path.read_text(encoding="utf-8")
+
+    @pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits are platform-specific")
+    def test_save_narrows_existing_broad_secret_file_mode(self):
+        # Simulate a pre-existing 0o644 file from before this hardening landed.
+        path = _subscriptions_path()
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(json.dumps({"old": {"secret": "stale", "prompt": "x"}}))
+        path.chmod(0o644)
+
+        _save_subscriptions({"demo": {"secret": "FRESH", "prompt": "x"}})
+
+        assert stat.S_IMODE(path.stat().st_mode) == 0o600
+        assert "FRESH" in path.read_text(encoding="utf-8")
+
 
 class TestWebhookEnabledGate:
     def test_blocks_when_disabled(self, capsys, monkeypatch):