feat(12-02): implement getNextNumber in DrizzlePlanRepository

- Import max from drizzle-orm for aggregate query
- Query MAX(number) where phase_id matches
- Return max + 1, or 1 if no plans exist
- Pattern follows DrizzlePhaseRepository.getNextNumber

src/db/repositories/drizzle/plan.ts

@@ -4,7 +4,7 @@
  * Implements PlanRepository interface using Drizzle ORM.
  */
 
-import { eq, asc } from 'drizzle-orm';
+import { eq, asc, max } from 'drizzle-orm';
 import { nanoid } from 'nanoid';
 import type { DrizzleDatabase } from '../../index.js';
 import { plans, type Plan } from '../../schema.js';
@@ -58,6 +58,16 @@ export class DrizzlePlanRepository implements PlanRepository {
       .orderBy(asc(plans.number));
   }
 
+  async getNextNumber(phaseId: string): Promise<number> {
+    const result = await this.db
+      .select({ maxNumber: max(plans.number) })
+      .from(plans)
+      .where(eq(plans.phaseId, phaseId));
+
+    const maxNumber = result[0]?.maxNumber ?? 0;
+    return maxNumber + 1;
+  }
+
   async update(id: string, data: UpdatePlanData): Promise<Plan> {
     const existing = await this.findById(id);
     if (!existing) {