fix(openclaw-plugin): implement context engine ingest/compact contract

[mobilebarn]

Summary

• ingest() now writes user/assistant messages to the OV session via addSessionMessage instead of returning a no-op { ingested: false }
• ingestBatch() delegates to ingest() for each message, accumulating the ingested count
• compact() commits the OV session as a fallback when delegated compaction is unavailable, ensuring the memory extraction pipeline still runs even without runtime compaction

Test plan

• Verify /compact command works when OpenViking is used as a context engine (previously returned no-op)
• Verify messages are ingested into OV sessions during normal conversation flow
• Verify batch ingestion correctly delegates to single-message ingest
• Verify compact fallback commits the OV session and logs appropriately
• Verify heartbeat and non-user/assistant messages are still skipped in ingest

Closes #833

[github-actions]

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