feat(gateway): SessionGCService three-tier GC + /gc command + cron (P8-014)

Implements three-tier garbage collection for agent sessions:
- SessionGCService.collect() for immediate per-session cleanup on destroySession()
- SessionGCService.sweepOrphans() for daily cron sweep of orphaned Valkey keys
- SessionGCService.fullCollect() for cold-start aggressive cleanup via OnModuleInit
- /gc slash command wired into CommandExecutorService + registered in CommandRegistryService
- SESSION_GC_CRON (daily 4am) added to CronService
- GCModule provides Valkey (ioredis via @mosaic/queue) and is imported by AgentModule, LogModule, CommandsModule, AppModule
- 8 Vitest unit tests covering all three GC tiers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/gateway/src/gc/session-gc.service.spec.ts

@@ -0,0 +1,97 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { Logger } from '@nestjs/common';
+import type { QueueHandle } from '@mosaic/queue';
+import type { LogService } from '@mosaic/log';
+import { SessionGCService } from './session-gc.service.js';
+
+type MockRedis = {
+  keys: ReturnType<typeof vi.fn>;
+  del: ReturnType<typeof vi.fn>;
+};
+
+describe('SessionGCService', () => {
+  let service: SessionGCService;
+  let mockRedis: MockRedis;
+  let mockLogService: { logs: { promoteToWarm: ReturnType<typeof vi.fn> } };
+
+  beforeEach(() => {
+    mockRedis = {
+      keys: vi.fn().mockResolvedValue([]),
+      del: vi.fn().mockResolvedValue(0),
+    };
+
+    mockLogService = {
+      logs: {
+        promoteToWarm: vi.fn().mockResolvedValue(0),
+      },
+    };
+
+    // Suppress logger output in tests
+    vi.spyOn(Logger.prototype, 'log').mockImplementation(() => {});
+
+    service = new SessionGCService(
+      mockRedis as unknown as QueueHandle['redis'],
+      mockLogService as unknown as LogService,
+    );
+  });
+
+  it('collect() deletes Valkey keys for session', async () => {
+    mockRedis.keys.mockResolvedValue(['mosaic:session:abc:system', 'mosaic:session:abc:foo']);
+    const result = await service.collect('abc');
+    expect(mockRedis.del).toHaveBeenCalledWith(
+      'mosaic:session:abc:system',
+      'mosaic:session:abc:foo',
+    );
+    expect(result.cleaned.valkeyKeys).toBe(2);
+  });
+
+  it('collect() with no keys returns empty cleaned valkeyKeys', async () => {
+    mockRedis.keys.mockResolvedValue([]);
+    const result = await service.collect('abc');
+    expect(result.cleaned.valkeyKeys).toBeUndefined();
+  });
+
+  it('collect() returns sessionId in result', async () => {
+    const result = await service.collect('test-session-id');
+    expect(result.sessionId).toBe('test-session-id');
+  });
+
+  it('fullCollect() deletes all session keys', async () => {
+    mockRedis.keys.mockResolvedValue(['mosaic:session:abc:system', 'mosaic:session:xyz:foo']);
+    const result = await service.fullCollect();
+    expect(mockRedis.del).toHaveBeenCalled();
+    expect(result.valkeyKeys).toBe(2);
+  });
+
+  it('fullCollect() with no keys returns 0 valkeyKeys', async () => {
+    mockRedis.keys.mockResolvedValue([]);
+    const result = await service.fullCollect();
+    expect(result.valkeyKeys).toBe(0);
+    expect(mockRedis.del).not.toHaveBeenCalled();
+  });
+
+  it('fullCollect() returns duration', async () => {
+    const result = await service.fullCollect();
+    expect(result.duration).toBeGreaterThanOrEqual(0);
+  });
+
+  it('sweepOrphans() extracts unique session IDs and collects them', async () => {
+    mockRedis.keys.mockResolvedValue([
+      'mosaic:session:abc:system',
+      'mosaic:session:abc:messages',
+      'mosaic:session:xyz:system',
+    ]);
+    mockRedis.del.mockResolvedValue(1);
+
+    const result = await service.sweepOrphans();
+    expect(result.orphanedSessions).toBeGreaterThanOrEqual(0);
+    expect(result.duration).toBeGreaterThanOrEqual(0);
+  });
+
+  it('sweepOrphans() returns empty when no session keys', async () => {
+    mockRedis.keys.mockResolvedValue([]);
+    const result = await service.sweepOrphans();
+    expect(result.orphanedSessions).toBe(0);
+    expect(result.totalCleaned).toHaveLength(0);
+  });
+});