feat(12-02): add getNextNumber to PlanRepository interface

- Add getNextNumber(phaseId) method signature
- Returns MAX(number) + 1, or 1 if no plans exist
- Following pattern from PhaseRepository.getNextNumber

src/db/repositories/plan-repository.ts

@@ -46,6 +46,12 @@ export interface PlanRepository {
    */
   findByPhaseId(phaseId: string): Promise<Plan[]>;
 
+  /**
+   * Get the next available plan number for a phase.
+   * Returns MAX(number) + 1, or 1 if no plans exist.
+   */
+  getNextNumber(phaseId: string): Promise<number>;
+
   /**
    * Update a plan.
    * Throws if plan not found.