refactor: migrate AgentService to use centralized logger

Replace console.error calls with createLogger for consistent logging across
the AgentService. This improves debuggability and makes logger calls testable.

Changes:
- Add createLogger import from @automaker/utils
- Add private logger instance initialized with 'AgentService' prefix
- Replace all 7 console.error calls with this.logger.error
- Update test mocks to use vi.hoisted() for proper mock access
- Update settings-helpers test to create mockLogger inside vi.mock()

Test Impact:
- All 774 tests passing
- Logger error calls are now verifiable in tests
- Mock logger properly accessible via vi.hoisted() pattern

Resolves Gemini Code Assist suggestions:
- "Make logger mockable for test assertions"
- "Use logger instead of console.error in AgentService"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/server/tests/unit/lib/settings-helpers.test.ts

@@ -5,14 +5,15 @@ import type { SettingsService } from '@/services/settings-service.js';
 // Mock the logger
 vi.mock('@automaker/utils', async () => {
   const actual = await vi.importActual('@automaker/utils');
+  const mockLogger = {
+    info: vi.fn(),
+    error: vi.fn(),
+    warn: vi.fn(),
+    debug: vi.fn(),
+  };
   return {
     ...actual,
-    createLogger: () => ({
-      info: vi.fn(),
-      error: vi.fn(),
-      warn: vi.fn(),
-      debug: vi.fn(),
-    }),
+    createLogger: () => mockLogger,
   };
 });