feat(ui): add agent creation choice modal and full-page config

Replace the direct agent config dialog with a choice modal offering two
paths: "Ask the CEO to create a new agent" (opens pre-filled issue) or
"I want advanced configuration myself" (navigates to /agents/new).

- Extend NewIssueDefaults with title/description for pre-fill support
- Add /agents/new route with full-page agent configuration form
- NewAgentDialog now shows CEO recommendation modal

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/context/DialogContext.tsx

@@ -5,6 +5,8 @@ interface NewIssueDefaults {
   priority?: string;
   projectId?: string;
   assigneeAgentId?: string;
+  title?: string;
+  description?: string;
 }
 
 interface NewGoalDefaults {