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()

assistant_migrations/script.py.mako

@@ -0,0 +1,26 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}

assistant_migrations/versions/001_assistant_pg_initial.py

@@ -0,0 +1,34 @@
+"""Initial PostgreSQL schema for assistant store (from SQLite parity DDL)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from alembic import op
+from sqlalchemy import text
+
+revision = "001_assistant_pg_initial"
+down_revision = None
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    bind = op.get_bind()
+    if bind.dialect.name != "postgresql":
+        return
+    root = Path(__file__).resolve().parents[2]
+    sql_path = root / "svc" / "persistence" / "ddl" / "postgresql_bootstrap.sql"
+    raw = sql_path.read_text(encoding="utf-8")
+    for part in raw.split(";"):
+        stmt = part.strip()
+        if not stmt or stmt.startswith("--"):
+            continue
+        op.execute(text(stmt))
+
+
+def downgrade() -> None:
+    bind = op.get_bind()
+    if bind.dialect.name != "postgresql":
+        return
+    raise NotImplementedError("Assistant PG downgrade not supported; restore from backup.")