feat(12-05): add spawnArchitectDecompose tRPC procedure

- Import buildDecomposePrompt from agent prompts
- Add spawnArchitectDecompose procedure for decompose mode
- Validates plan and phase exist before spawning
- Spawns agent with decompose mode and comprehensive prompt

src/trpc/router.ts

@@ -17,7 +17,7 @@ import type { PlanRepository } from '../db/repositories/plan-repository.js';
 import type { DispatchManager } from '../dispatch/types.js';
 import type { CoordinationManager } from '../coordination/types.js';
 import type { Phase, Plan, Task } from '../db/schema.js';
-import { buildDiscussPrompt, buildBreakdownPrompt } from '../agent/prompts.js';
+import { buildDiscussPrompt, buildBreakdownPrompt, buildDecomposePrompt } from '../agent/prompts.js';
 
 /**
  * Initialize tRPC with our context type.
@@ -1056,6 +1056,49 @@ export const appRouter = router({
         mode: 'breakdown',
       });
     }),
+
+  /**
+   * Spawn architect in decompose mode.
+   * Uses comprehensive decompose prompt to break a plan into executable tasks.
+   */
+  spawnArchitectDecompose: publicProcedure
+    .input(z.object({
+      name: z.string().min(1),
+      planId: z.string().min(1),
+      context: z.string().optional(),
+    }))
+    .mutation(async ({ ctx, input }) => {
+      const agentManager = requireAgentManager(ctx);
+      const planRepo = requirePlanRepository(ctx);
+      const phaseRepo = requirePhaseRepository(ctx);
+
+      // 1. Get plan and its phase
+      const plan = await planRepo.findById(input.planId);
+      if (!plan) {
+        throw new TRPCError({
+          code: 'NOT_FOUND',
+          message: `Plan '${input.planId}' not found`,
+        });
+      }
+      const phase = await phaseRepo.findById(plan.phaseId);
+      if (!phase) {
+        throw new TRPCError({
+          code: 'NOT_FOUND',
+          message: `Phase '${plan.phaseId}' not found`,
+        });
+      }
+
+      // 2. Build decompose prompt
+      const prompt = buildDecomposePrompt(plan, phase, input.context);
+
+      // 3. Spawn agent in decompose mode
+      return agentManager.spawn({
+        name: input.name,
+        taskId: input.planId, // Associate with plan
+        prompt,
+        mode: 'decompose',
+      });
+    }),
 });
 
 /**