Fix(migration): repair CSGClaw agent participants

[xxx7xxxx]

Check your own machine:

jq '.participants[] | {id, agent_id, channel_user_ref}' ~/.csgclaw/im/participants.json
jq '.channels.csgclaw.participant_id' ~/.csgclaw/agents/<agent-name>/.picoclaw/config.json

If that shows id: "u-xxx" but config says "xxx", that is the break.

Summary:

• Repair participant store migration for legacy CSGClaw worker agents by converting old u-* participant IDs to stripped participant IDs while preserving agent_id and channel_user_ref.
• Persist the repair on store load so both fresh bots.json imports and already-written stale participants.json files are fixed.