fix(paired): cap consecutive reviewer silent-failure retries

The reviewer (codex) sometimes emits a final result with no visible text.
The runtime marks the run failed → handleFailedReviewerExecution
preserves status as review_ready → the follow-up scheduler re-queues
another reviewer-turn → loop. The existing round_trip / arbiter caps
never fire because round_trip_count only advances on owner-side
submissions; bot-side reviewer flakes never increment it.

Mirror the owner_failure_count pattern: add reviewer_failure_count to
paired_tasks, increment on each silent failure, and escalate via
requestArbiterOrEscalate once it reaches 2. Reset to 0 on every
successful reviewer completion path (PROCEED/REVISE/arbiter/wait-for-user)
and when the owner re-submits a fresh review cycle.

Repro chat: 1507762222724546560 (stock-adiviser). Task
33968d31-0da2-480c-85d3-7a3999822ab4 logged 11 consecutive
reviewer-turn entries with no work_items and round_trip_count stuck
at 1.

src/db/migrations/index.ts

@@ -14,6 +14,7 @@ import { OWNER_FAILURE_COUNT_MIGRATION } from './011_owner-failure-count.js';
 import { PAIRED_VERDICT_AND_STEP_TELEMETRY_MIGRATION } from './012_paired-verdict-and-step-telemetry.js';
 import { MESSAGE_SOURCE_KIND_MIGRATION } from './013_message-source-kind.js';
 import { WORK_ITEM_ATTACHMENTS_MIGRATION } from './014_work-item-attachments.js';
+import { REVIEWER_FAILURE_COUNT_MIGRATION } from './015_reviewer-failure-count.js';
 import type {
   SchemaMigrationArgs,
   SchemaMigrationDefinition,
@@ -36,6 +37,7 @@ const ORDERED_SCHEMA_MIGRATIONS: readonly SchemaMigrationDefinition[] = [
   PAIRED_VERDICT_AND_STEP_TELEMETRY_MIGRATION,
   MESSAGE_SOURCE_KIND_MIGRATION,
   WORK_ITEM_ATTACHMENTS_MIGRATION,
+  REVIEWER_FAILURE_COUNT_MIGRATION,
 ];
 
 function ensureSchemaMigrationsTable(database: Database): void {