test: add unit/integration/E2E test suites, fix create_session bug, update docs

- Unit tests: test_session.py (27), test_error_kb.py (24), test_agent.py hardened
- Integration tests: test_api_integration.py (25) with FastAPI TestClient
- E2E tests: main-flows.spec.ts (8) with Playwright + API mocking
- Bug fix: backend/session.py create_session() missing session_id parameter
- Config: frontend/playwright.config.ts, npm run test:e2e
- Docs: update CLAUDE.md v9, .gitignore for test artifacts/eval reports

frontend/playwright.config.ts

@@ -0,0 +1,20 @@
+import { defineConfig } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./tests/e2e",
+  timeout: 60000,
+  expect: { timeout: 10000 },
+  retries: 0,
+  use: {
+    baseURL: "http://localhost:5173",
+    headless: true,
+    screenshot: "only-on-failure",
+    trace: "retain-on-failure",
+  },
+  webServer: {
+    command: "npm run dev",
+    url: "http://localhost:5173",
+    reuseExistingServer: true,
+    timeout: 30000,
+  },
+});