feat: Remove checkpoint task types — per-phase review is sufficient

Checkpoint tasks (human-verify, decision, human-action) silently blocked
auto-dispatch with no UI to resolve them. Per-phase review + initiative
review already cover human verification, making checkpoints redundant.

Removed from: schema, dispatch manager, tRPC validators, detail prompt,
frontend types, tests, and docs.

apps/server/db/repositories/drizzle/task.test.ts

@@ -71,13 +71,13 @@ describe('DrizzleTaskRepository', () => {
     it('should accept custom type and priority', async () => {
       const task = await taskRepo.create({
         phaseId: testPhaseId,
-        name: 'Checkpoint Task',
-        type: 'checkpoint:human-verify',
+        name: 'High Priority Task',
+        type: 'auto',
         priority: 'high',
         order: 1,
       });
 
-      expect(task.type).toBe('checkpoint:human-verify');
+      expect(task.type).toBe('auto');
       expect(task.priority).toBe('high');
     });
   });