feat: Add retry mechanism for blocked tasks

Blocked tasks (from spawn failures) were a dead-end with no way to
recover. Add retryBlockedTask to DispatchManager that resets status
to pending and re-queues, a tRPC mutation that also kicks dispatchNext,
and a Retry button in the task slide-over when status is blocked.

docs/dispatch-events.md

@@ -68,6 +68,7 @@ InitiativeReviewApprovedEvent  { initiativeId, branch, strategy: 'push_branch' |
 6. **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`.
 
 ### DispatchManager Methods
 
@@ -78,6 +79,7 @@ InitiativeReviewApprovedEvent  { initiativeId, branch, strategy: 'push_branch' |
 | `getNextDispatchable()` | Get next task without dispatching |
 | `completeTask(taskId, agentId?)` | Complete task |
 | `blockTask(taskId, reason)` | Block task with reason |
+| `retryBlockedTask(taskId)` | Reset blocked task to pending and re-queue |
 | `getQueueState()` | Return queued, ready, blocked tasks |
 
 ## Phase Dispatch