feat(KB-633): add Mission REST API routes for real-time steering

- Create mission-routes.ts with comprehensive REST API for task steering
- Add GET/PUT /api/tasks/:id/mission endpoints for mission state management
- Add POST endpoints for steering comments, pause/unpause, retry, and cancel
- Mount mission routes in dashboard API server
- Update core store with mission-related functionality
- Refactor executor: remove old mission logic, update tests

packages/dashboard/src/routes.ts

@@ -20,6 +20,7 @@ import {
   hasIssueBadgeFieldsChanged,
   type BadgeUrlComponents,
 } from "./github-webhooks.js";
+import { createMissionRouter } from "./mission-routes.js";
 
 /**
  * Minimal interface matching pi-coding-agent's ModelRegistry API surface
@@ -5664,6 +5665,10 @@ Output ONLY the prompt text (no markdown, no explanations).`;
     }
   });
 
+  // ── Mission Routes ─────────────────────────────────────────────────────────
+  // Mount mission routes at /api/missions
+  router.use("/missions", createMissionRouter(store));
+
   return router;
 }