feat: add --unassigned flag to bd ready command

Adds the ability to filter ready work for issues with no assignee,
which is useful for the SCAVENGE protocol in Gas Town where polecats
need to query the "Salvage Yard" for unclaimed work.

Changes:
- Add Unassigned bool field to types.WorkFilter
- Add --unassigned/-u flag to bd ready command
- Update SQL query in GetReadyWork to filter for NULL/empty assignee
- Add Unassigned field to RPC ReadyArgs for daemon support
- Add tests for the new functionality

Closes: gt-3rp

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

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

internal/types/types.go

@@ -351,6 +351,7 @@ type WorkFilter struct {
 	Status     Status
 	Priority   *int
 	Assignee   *string
+	Unassigned bool       // Filter for issues with no assignee
 	Labels     []string   // AND semantics: issue must have ALL these labels
 	LabelsAny  []string   // OR semantics: issue must have AT LEAST ONE of these labels
 	Limit      int