perf: split monolithic E2E test into 8 parallel files

Split test/skill-e2e.test.ts (3442 lines) into 8 category files:
- skill-e2e-browse.test.ts (7 tests)
- skill-e2e-review.test.ts (7 tests)
- skill-e2e-qa-bugs.test.ts (3 tests)
- skill-e2e-qa-workflow.test.ts (4 tests)
- skill-e2e-plan.test.ts (6 tests)
- skill-e2e-design.test.ts (7 tests)
- skill-e2e-workflow.test.ts (6 tests)
- skill-e2e-deploy.test.ts (4 tests)

Bun runs each file in its own worker = 10 parallel workers
(8 split + routing + codex). Expected: 78 min → ~12 min.

Extracted shared helpers to test/helpers/e2e-helpers.ts.

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

ARCHITECTURE.md

@@ -284,7 +284,7 @@ The `parseNDJSON()` function is pure — no I/O, no side effects — making it i
 ### Observability data flow
 
 ```
-  skill-e2e.test.ts
+  skill-e2e-*.test.ts
         │
         │ generates runId, passes testName + runId to each call
         │