Remove task-level approval system

Task-level approval (requiresApproval, mergeRequiresApproval,
pending_approval status) was redundant with executionMode
(yolo vs review_per_phase) and blocked the orchestrator's
phase completion flow. Tasks now complete directly;
phase-level review via executionMode is the right granularity.

Removed: schema columns (left in DB, removed from Drizzle),
TaskPendingApprovalEvent, approveTask/listPendingApprovals
procedures, findPendingApproval repository method, and all
frontend approval UI.

apps/server/trpc/routers/initiative.ts

@@ -204,7 +204,6 @@ export function initiativeProcedures(publicProcedure: ProcedureBuilder) {
     updateInitiativeConfig: publicProcedure
       .input(z.object({
         initiativeId: z.string().min(1),
-        mergeRequiresApproval: z.boolean().optional(),
         executionMode: z.enum(['yolo', 'review_per_phase']).optional(),
         branch: z.string().nullable().optional(),
       }))