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>

cmd/bd/ready.go

@@ -39,7 +39,7 @@ var readyCmd = &cobra.Command{
 			priority, _ := cmd.Flags().GetInt("priority")
 			filter.Priority = &priority
 		}
-		if assignee != "" {
+		if assignee != "" && !unassigned {
 			filter.Assignee = &assignee
 		}
 		// Validate sort policy