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/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.")