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/database.md

@@ -44,7 +44,7 @@ All adapters use nanoid() for IDs, auto-manage timestamps, and use Drizzle's `.r
 | parentTaskId | text nullable self-ref FK (cascade) | decomposition hierarchy |
 | name | text NOT NULL | |
 | description | text nullable | |
-| type | text enum | 'auto' \| 'checkpoint:human-verify' \| 'checkpoint:decision' \| 'checkpoint:human-action' |
+| type | text enum | 'auto' |
 | category | text enum | 'execute' \| 'research' \| 'discuss' \| 'plan' \| 'detail' \| 'refine' \| 'verify' \| 'merge' \| 'review' |
 | priority | text enum | 'low' \| 'medium' \| 'high' |
 | status | text enum | 'pending' \| 'in_progress' \| 'completed' \| 'blocked' |