fix(sessions): delete on-disk transcript files during prune and delete (#3015)

`delete_session()` and `prune_sessions()` only removed SQLite records,
leaving .json/.jsonl transcript files on disk forever. Over time this
causes unbounded disk growth (~27MB/day observed).

Changes:
- Add `_remove_session_files()` static helper that cleans up
  `{session_id}.json`, `.jsonl`, and `request_dump_{session_id}_*.json`
- `delete_session()` accepts optional `sessions_dir` param and removes
  files for the deleted session and its children
- `prune_sessions()` accepts optional `sessions_dir` param and removes
  files for all pruned sessions after the DB transaction
- Wire up CLI `hermes sessions delete` and `hermes sessions prune` to
  pass `sessions_dir`
- File cleanup is best-effort (OSError silenced) so DB operations are
  never blocked by filesystem issues
- Fully backward-compatible: `sessions_dir=None` (default) preserves
  existing behavior

gateway/run.py

@@ -763,6 +763,7 @@ class GatewayRunner:
                         retention_days=int(_sess_cfg.get("retention_days", 90)),
                         min_interval_hours=int(_sess_cfg.get("min_interval_hours", 24)),
                         vacuum=bool(_sess_cfg.get("vacuum_after_prune", True)),
+                        sessions_dir=self.config.sessions_dir,
                     )
             except Exception as exc:
                 logger.debug("state.db auto-maintenance skipped: %s", exc)