feat(orchestrator): build reliability features — git integrity, reserved filename cleanup, archival

Add three optional reliability features for hardening CI pipelines:
- Git corruption detection & recovery (fsck, stale lock cleanup,
  submodule backing store validation, auto-recovery)
- Reserved filename cleanup (removes Windows device names that
  cause Unity asset importer infinite loops)
- Build output archival with configurable retention policy

All features are opt-in and fail gracefully with warnings only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/model/orchestrator/services/reliability/build-reliability-service.test.ts

@@ -0,0 +1,54 @@
+import fs from 'node:fs';
+import { BuildReliabilityService } from './build-reliability-service';
+
+// Mock dependencies
+jest.mock('node:fs');
+jest.mock('../core/orchestrator-logger');
+jest.mock('../core/orchestrator-system');
+
+const mockFs = fs as jest.Mocked<typeof fs>;
+
+describe('BuildReliabilityService', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  describe('cleanStaleLockFiles', () => {
+    it('should return 0 when .git directory does not exist', async () => {
+      mockFs.existsSync.mockReturnValue(false);
+      const result = await BuildReliabilityService.cleanStaleLockFiles('/repo');
+      expect(result).toBe(0);
+    });
+  });
+
+  describe('cleanReservedFilenames', () => {
+    it('should return empty array when Assets directory does not exist', async () => {
+      mockFs.existsSync.mockReturnValue(false);
+      const result = await BuildReliabilityService.cleanReservedFilenames('/project');
+      expect(result).toEqual([]);
+    });
+  });
+
+  describe('validateSubmoduleBackingStores', () => {
+    it('should return empty array when .gitmodules does not exist', async () => {
+      mockFs.existsSync.mockReturnValue(false);
+      const result = await BuildReliabilityService.validateSubmoduleBackingStores('/repo');
+      expect(result).toEqual([]);
+    });
+  });
+
+  describe('enforceRetention', () => {
+    it('should return 0 when archive path does not exist', async () => {
+      mockFs.existsSync.mockReturnValue(false);
+      const result = await BuildReliabilityService.enforceRetention('/archive', 3);
+      expect(result).toBe(0);
+    });
+  });
+
+  describe('configureGitEnvironment', () => {
+    it('should return GIT_CONFIG_NOSYSTEM=1', () => {
+      const environment = BuildReliabilityService.configureGitEnvironment();
+      expect(environment.GIT_CONFIG_NOSYSTEM).toBe('1');
+    });
+  });
+});