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>

gateway/platforms/api_server.py

@@ -35,6 +35,7 @@ import re
 import sqlite3
 import time
 import uuid
+from pathlib import Path
 from typing import Any, Dict, List, Optional
 
 try:
@@ -337,10 +338,12 @@ class ResponseStore:
                 db_path = str(get_hermes_home() / "response_store.db")
             except Exception:
                 db_path = ":memory:"
+        self._db_path: Optional[str] = db_path if db_path != ":memory:" else None
         try:
             self._conn = sqlite3.connect(db_path, check_same_thread=False)
         except Exception:
             self._conn = sqlite3.connect(":memory:", check_same_thread=False)
+            self._db_path = None
         # Use shared WAL-fallback helper so response_store.db degrades
         # gracefully on NFS/SMB/FUSE-mounted HERMES_HOME (same filesystem
         # issue addressed for state.db/kanban.db — see
@@ -361,6 +364,31 @@ class ResponseStore:
             )"""
         )
         self._conn.commit()
+        # response_store.db contains conversation history (tool payloads,
+        # prompts, results). Tighten to owner-only after creation so other
+        # local users on a shared box can't read it. Run once at __init__
+        # rather than after every commit — chmod-on-every-write is wasted
+        # syscalls on a hot path.
+        self._tighten_file_permissions()
+
+    def _tighten_file_permissions(self) -> None:
+        """Force owner-only permissions on the DB and SQLite sidecars."""
+        if not self._db_path:
+            return
+        for candidate in (
+            Path(self._db_path),
+            Path(f"{self._db_path}-wal"),
+            Path(f"{self._db_path}-shm"),
+        ):
+            try:
+                if candidate.exists():
+                    candidate.chmod(0o600)
+            except OSError:
+                logger.debug(
+                    "Failed to restrict response store permissions for %s",
+                    candidate,
+                    exc_info=True,
+                )
 
     def get(self, response_id: str) -> Optional[Dict[str, Any]]:
         """Retrieve a stored response by ID (updates access time for LRU)."""