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/trpc/routers/phase-dispatch.ts

@@ -53,7 +53,7 @@ export function phaseDispatchProcedures(publicProcedure: ProcedureBuilder) {
           number: z.number().int().positive(),
           name: z.string().min(1),
           description: z.string(),
-          type: z.enum(['auto', 'checkpoint:human-verify', 'checkpoint:decision', 'checkpoint:human-action']).default('auto'),
+          type: z.enum(['auto']).default('auto'),
           dependencies: z.array(z.number().int().positive()).optional(),
         })),
       }))