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/rpc/server_issues_epics.go

@@ -961,7 +961,7 @@ func (s *Server) handleReady(req *Request) Response {
 		Labels:     util.NormalizeLabels(readyArgs.Labels),
 		LabelsAny:  util.NormalizeLabels(readyArgs.LabelsAny),
 	}
-	if readyArgs.Assignee != "" {
+	if readyArgs.Assignee != "" && !readyArgs.Unassigned {
 		wf.Assignee = &readyArgs.Assignee
 	}