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/src/rejection-handler/interfaces/escalation.interface.ts

@@ -0,0 +1,13 @@
+export interface EscalationRule {
+  condition: "max-attempts" | "critical-failure" | "time-exceeded";
+  action: "notify" | "reassign" | "block" | "cancel";
+  target?: string; // notification target
+  priority: "low" | "medium" | "high" | "critical";
+}
+
+export interface EscalationConfig {
+  rules: EscalationRule[];
+  notifyOnRejection: boolean;
+  autoReassign: boolean;
+  maxWaitTime: number; // minutes before auto-escalation
+}

apps/api/src/rejection-handler/interfaces/index.ts

@@ -0,0 +1,2 @@
+export * from "./rejection.interface";
+export * from "./escalation.interface";

apps/api/src/rejection-handler/interfaces/rejection.interface.ts

@@ -0,0 +1,25 @@
+export interface RejectionContext {
+  taskId: string;
+  workspaceId: string;
+  agentId: string;
+  attemptCount: number;
+  failures: FailureSummary[];
+  originalTask: string;
+  startedAt: Date;
+  rejectedAt: Date;
+}
+
+export interface FailureSummary {
+  gateName: string;
+  failureType: string;
+  message: string;
+  attempts: number;
+}
+
+export interface RejectionResult {
+  handled: boolean;
+  escalated: boolean;
+  notificationsSent: string[];
+  taskState: "blocked" | "reassigned" | "cancelled";
+  manualReviewRequired: boolean;
+}