Add -u/--unassigned flag to bd ready command to show only issues
with no assignee. This supports the SCAVENGE protocol where polecats
query the 'Salvage Yard' for unassigned ready work.
Changes:
- Add NoAssignee field to WorkFilter struct
- Update SQLite GetReadyWork to filter by empty/null assignee
- Add --unassigned/-u flag to ready command
- Update RPC protocol and daemon handler
- Add test for NoAssignee filter functionality
Fixes: gt-3rp
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Co-Authored-By: Claude <noreply@anthropic.com>