`pg_session_store.py` mixed three layers: schema DDL and migrations, the
row <-> `Message` mapping with its archive-aware hydration, and the store
itself. Split them into `navi/core/sessions/`:

* `ddl.py` — table/index definitions and the one-shot migrations;
* `rows.py` — serialization, dangling-tool-call repair, session hydration;
* `store.py` — `PgSessionStore` and the pending-sweep loop.

`navi/core/pg_session_store.py` stays as a compatibility shim re-exporting
both the public surface and the private helpers the tests use. No behavior
change: every top-level function and every store method is preserved.