feat(ready): add --unassigned filter for bd ready

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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/storage/sqlite/ready.go

@@ -29,6 +29,7 @@ func (s *SQLiteStorage) GetReadyWork(ctx context.Context, filter types.WorkFilte
 		args = append(args, *filter.Priority)
 	}
 
+	// Unassigned takes precedence over Assignee filter
 	if filter.Unassigned {
 		whereClauses = append(whereClauses, "(i.assignee IS NULL OR i.assignee = '')")
 	} else if filter.Assignee != nil {