episodes extract-sessions: add --follow (responsive extract)

[phenomenoner]

Background

• We now support openclaw-mem episodes ingest --follow (tailer) with rotate-on-idle + spool lock.
• Extraction is still driven by a 1m cron that scans sessions and appends an episodes JSONL spool.

Request

• Add first-class responsive extract mode, e.g.
  • openclaw-mem episodes extract-sessions --follow
  • or a dedicated episodes extract-follow subcommand.

Goals

• Reduce dependence on cron for low-latency demo / product-grade “responsive capture”.
• Bounded CPU when idle.
• State/offset resume across restarts.
• Safe coexistence with follow ingest + rotate-on-idle (spool lock coordination).

Proposed UX

• --follow + --poll-interval-ms
• --idle-exit-seconds (optional for supervised jobs/tests)
• --sessions-root scan strategy should be explicit (glob, include/exclude, max-bytes per file).

Acceptance

• Integration test that appends to a session JSONL and asserts new spool lines emitted.
• Docs update: recommended architecture (extract follow + ingest follow).

Notes

• Keep 1m cron as fallback; follow mode becomes the recommended default for showcase deployments.