feat(#166): Implement Stitcher module structure

Created the mosaic-stitcher module - the workflow orchestration layer
that wraps OpenClaw.

Responsibilities:
- Receive webhooks from @mosaic bot
- Apply Guard Rails (capability permissions)
- Apply Quality Rails (mandatory gates)
- Track all job steps and events
- Dispatch work to OpenClaw with constraints

Implementation:
- StitcherModule: Module definition with PrismaModule and BullMqModule
- StitcherService: Core orchestration logic
  - handleWebhook(): Process webhooks from @mosaic bot
  - dispatchJob(): Create RunnerJob and dispatch to BullMQ queue
  - applyGuardRails(): Check capability permissions for agent profiles
  - applyQualityRails(): Determine mandatory gates for job types
  - trackJobEvent(): Log events to database for audit trail
- StitcherController: HTTP endpoints
  - POST /stitcher/webhook: Webhook receiver
  - POST /stitcher/dispatch: Manual job dispatch
- DTOs and interfaces for type safety

TDD Process:
1. RED: Created failing tests (12 tests)
2. GREEN: Implemented minimal code to pass tests
3. REFACTOR: Fixed TypeScript strict mode issues

Quality Gates: ALL PASS
- Typecheck: PASS
- Lint: PASS
- Build: PASS
- Tests: PASS (12/12)

Token estimate: ~56,000 tokens

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

apps/api/src/stitcher/dto/index.ts

@@ -0,0 +1 @@
+export * from "./webhook.dto";

apps/api/src/stitcher/dto/webhook.dto.ts

@@ -0,0 +1,44 @@
+import { IsString, IsUUID, IsOptional, IsObject, ValidateNested } from "class-validator";
+import { Type } from "class-transformer";
+
+/**
+ * DTO for webhook payload from @mosaic bot
+ */
+export class WebhookPayloadDto {
+  @IsString()
+  issueNumber!: string;
+
+  @IsString()
+  repository!: string;
+
+  @IsString()
+  action!: string; // 'assigned', 'mentioned', 'commented'
+
+  @IsOptional()
+  @IsString()
+  comment?: string;
+
+  @IsOptional()
+  @IsObject()
+  metadata?: Record<string, unknown>;
+}
+
+/**
+ * DTO for dispatching a job
+ */
+export class DispatchJobDto {
+  @IsUUID("4")
+  workspaceId!: string;
+
+  @IsString()
+  type!: string; // 'git-status', 'code-task', 'priority-calc'
+
+  @IsOptional()
+  @ValidateNested()
+  @Type(() => WebhookPayloadDto)
+  webhookPayload?: WebhookPayloadDto;
+
+  @IsOptional()
+  @IsObject()
+  context?: Record<string, unknown>;
+}