refactor(e2e): extract infrastructure into tests/e2e/infra/ module

- Move fixtures.ts, llm-judge.ts, results-db.ts to tests/e2e/infra/
- Deduplicate wdio.conf.js: use createTestFixture() instead of inline copy
- Replace __dirname paths with projectRoot-anchored paths
- Create test-mode-constants.ts (typed env var names, flag registry)
- Create scripts/preflight-check.sh (validates tauri-driver, display, Claude CLI)
- Create scripts/check-test-flags.sh (CI lint for AGOR_TEST flag drift)
- Rewrite tests/e2e/README.md with full documentation
- Update spec imports for moved infra files

tests/e2e/specs/phase-c.test.ts

@@ -1,5 +1,5 @@
 import { browser, expect } from '@wdio/globals';
-import { isJudgeAvailable, assertWithJudge } from '../llm-judge';
+import { isJudgeAvailable, assertWithJudge } from '../infra/llm-judge';
 
 // Phase C: Hardening feature tests.
 // Tests the v3 production-readiness features added in the hardening sprint: