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/gateway/test_api_server.py

@@ -14,6 +14,8 @@ Tests cover:
 
 import asyncio
 import json
+import os
+import stat
 import time
 import uuid
 from unittest.mock import AsyncMock, MagicMock, patch
@@ -128,6 +130,37 @@ class TestResponseStore:
         # resp_2 mapping should still be intact
         assert store.get_conversation("chat-b") == "resp_2"
 
+    @pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits are platform-specific")
+    def test_file_store_created_owner_only_under_permissive_umask(self, tmp_path):
+        """response_store.db must be 0o600 on creation even under umask 022."""
+        db_path = tmp_path / "response_store.db"
+        store = None
+        old_umask = os.umask(0o022)
+        try:
+            store = ResponseStore(max_size=10, db_path=str(db_path))
+            store.put(
+                "resp_secret",
+                {
+                    "response": {"id": "resp_secret"},
+                    "conversation_history": [{"role": "tool", "content": "dummy-marker"}],
+                },
+            )
+        finally:
+            os.umask(old_umask)
+            if store is not None:
+                store.close()
+
+        assert stat.S_IMODE(db_path.stat().st_mode) == 0o600
+        # WAL/SHM sidecars are owner-only too when present. WAL mode may be
+        # unavailable on some filesystems (NFS/SMB) — only assert when the
+        # sidecar files actually exist.
+        for sidecar in (
+            db_path.with_name(db_path.name + "-wal"),
+            db_path.with_name(db_path.name + "-shm"),
+        ):
+            if sidecar.exists():
+                assert stat.S_IMODE(sidecar.stat().st_mode) == 0o600
+
 
 # ---------------------------------------------------------------------------
 # _IdempotencyCache