…rowth

SessionDB.close() was never called in practice, causing SQLite WAL files to
grow unboundedly on long-running gateway and CLI processes (field reports of
100+ MB WAL files).

- Add `atexit.register(self.close)` in SessionDB.__init__ so the WAL
  checkpoint always runs on process exit, even without an explicit close()
- Add __enter__/__exit__ so SessionDB can be used as a context manager
- Fix the silent `except Exception: pass` in close() to log a warning for
  non-ENOENT errors, aiding operator diagnostics
- Add explicit close() call in the CLI atexit cleanup path as a belt-and-
  suspenders measure alongside the atexit registration
- Add TestWALCheckpointOnClose test class (6 tests) covering: WAL file is
  empty/absent after close, idempotent close, context manager closes on
  normal and exceptional exit, atexit registration present in source, and
  PRAGMA wal_checkpoint return value verification

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>