feat(#139): build Gate Rejection Response Handler

Implement rejection handling for tasks that fail quality gates after
all continuation attempts are exhausted.

Schema:
- Add TaskRejection model for tracking rejections
- Store failures, attempts, escalation state

Service:
- handleRejection: Main entry point for rejection handling
- logRejection: Database logging
- determineEscalation: Rule-based escalation determination
- executeEscalation: Execute escalation actions
- sendNotification: Notification dispatch
- markForManualReview: Flag tasks for human review
- getRejectionHistory: Query rejection history
- generateRejectionReport: Markdown report generation

Escalation rules:
- max-attempts: Trigger after 3+ attempts
- time-exceeded: Trigger after 2+ hours
- critical-failure: Trigger on security/critical issues

Actions: notify, block, reassign, cancel

Tests: 16 passing with 80% statement coverage

Fixes #139

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/prisma/schema.prisma

@@ -1030,3 +1030,26 @@ model QualityGate {
   @@index([workspaceId, isEnabled])
   @@map("quality_gates")
 }
+
+model TaskRejection {
+  id            String    @id @default(uuid()) @db.Uuid
+  taskId        String    @map("task_id")
+  workspaceId   String    @map("workspace_id")
+  agentId       String    @map("agent_id")
+  attemptCount  Int       @map("attempt_count")
+  failures      Json      // FailureSummary[]
+  originalTask  String    @map("original_task")
+  startedAt     DateTime  @map("started_at") @db.Timestamptz
+  rejectedAt    DateTime  @map("rejected_at") @db.Timestamptz
+  escalated     Boolean   @default(false)
+  manualReview  Boolean   @default(false) @map("manual_review")
+  resolvedAt    DateTime? @map("resolved_at") @db.Timestamptz
+  resolution    String?
+
+  @@index([taskId])
+  @@index([workspaceId])
+  @@index([agentId])
+  @@index([escalated])
+  @@index([manualReview])
+  @@map("task_rejections")
+}