feat: timestamped GBrain shadow contribution log for Cortex parity

[100yenadmin]

Summary

Add timestamped observe-only contribution logs for GBrain so we can compare same-day parity against Cortex.

Log requirements

For each shadow contribution, record:

• timestamp
• session/conversation identifier if available
• trigger type (signal-detect, retrieval-candidate, end-of-turn capture, identity-file draft, etc.)
• source surface
• what GBrain would have contributed
• whether contribution was suppressed / skipped
• correlation id or run id for grouping

Storage requirements

• Keep logs in a separate shadow log surface, not mixed into authoritative Cortex memory
• Easy to diff against Cortex output later
• Exportable for 7-day review

Acceptance criteria

• Observe-only logging exists for all enabled GBrain shadow paths
• Log entries are timestamped and structured
• Logs can be queried by day/session/feature
• No user-visible behavior changes are forced by these logs

Notes

This is the core parity mechanism for v1.