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.

docs/dispatch-events.md

@@ -64,8 +64,7 @@ InitiativeReviewApprovedEvent  { initiativeId, branch, strategy: 'push_branch' |
 2. **Dispatch** — `dispatchNext()` finds highest-priority task with all deps complete
 3. **Context gathering** — Before spawn, `dispatchNext()` gathers initiative context (initiative, phase, tasks, pages) and passes as `inputContext` to the agent. Agents receive `.cw/input/task.md`, `initiative.md`, `phase.md`, `context/tasks/`, `context/phases/`, and `pages/`.
 4. **Priority order**: high > medium > low, then oldest first (FIFO within priority)
-5. **Checkpoint skip** — Tasks with type starting with `checkpoint:` skip auto-dispatch
-6. **Planning skip** — Planning-category tasks (research, discuss, plan, detail, refine) skip auto-dispatch — they use the architect flow
+5. **Planning skip** — Planning-category tasks (research, discuss, plan, detail, refine) skip auto-dispatch — they use the architect flow
 7. **Summary propagation** — `completeTask()` reads the completing agent's `result.message` and stores it on the task's `summary` column. Dependent tasks see this summary in `context/tasks/<id>.md` frontmatter.
 8. **Spawn failure** — If `agentManager.spawn()` throws, the task is blocked via `blockTask()` with the error message. The dispatch cycle continues instead of crashing.
 9. **Retry blocked** — `retryBlockedTask(taskId)` resets a blocked task to pending and re-queues it. Exposed via tRPC `retryBlockedTask` mutation. The UI shows a Retry button in the task slide-over when status is `blocked`.