feat(persistence): PostgreSQL assistant store, chat persist fixes, gateway scripts

- Add SQLAlchemy Core repos, pg adapter/compat, assistant_store factory, Alembic bootstrap and migration/cutover scripts.

- Harden chat_message writes (NUL scrub for PG), turn_uuid on attempt failure, WS turn_runner fallbacks and gateway executed_turn_uuid init.

- start_gateway: log paths, PS7 stderr handling via cmd, background stdout/stderr redirect; runtime assistant_runtime_log_dir export.

- Ops: clear_all_chat_sessions with PG-only --postgresql and env-gated wipe; clear_postgres_chat_sessions.ps1.

- Tests: SA repos, pg compat, persist fallback, smoke env isolation; CI and docs touch-ups.

Co-authored-by: Cursor <cursoragent@cursor.com>

assistant_migrations/env.py

@@ -0,0 +1,67 @@
+"""Alembic environment (assistant DB URL from env)."""
+
+from __future__ import annotations
+
+import os
+import sys
+from logging.config import fileConfig
+from pathlib import Path
+
+from sqlalchemy import engine_from_config, pool, text
+from sqlalchemy.engine import Connection
+
+from alembic import context
+
+ROOT = Path(__file__).resolve().parents[1]
+if str(ROOT) not in sys.path:
+    sys.path.insert(0, str(ROOT))
+
+os.chdir(ROOT)
+
+from svc.config.bootstrap_env import load_system_env  # noqa: E402
+
+load_system_env()
+
+from svc.config.database import assistant_db_backend, assistant_sqlalchemy_url  # noqa: E402
+
+config = context.config
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = None
+
+
+def get_url() -> str:
+    return assistant_sqlalchemy_url()
+
+
+def run_migrations_offline() -> None:
+    context.configure(
+        url=get_url(),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    ini_section = config.get_section(config.config_ini_section) or {}
+    ini_section["sqlalchemy.url"] = get_url()
+    connectable = engine_from_config(
+        ini_section,
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    with connectable.connect() as connection:
+        context.configure(connection=connection, target_metadata=target_metadata)
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()