Custom Engine e2e: assert generated report content, not just summary substring

[zpzjzj]

Background

The Custom Engine e2e tests (e2e/custom_engine_test.go) currently assert only that the CLI produced output containing the substrings PASS and 1 passed. They do not verify:

• The custom engine's final_message actually reached the judge.
• expect.must_contain and other gate assertions evaluated correctly.
• The generated report.json payload matches what was actually executed.

A custom engine that returns the wrong final_message but happens to print PASS somewhere would pass these tests.

Proposal

After the run, parse the generated report.json and assert:

• The case's final_message matches the fixture script's output.
• The case's judge.outcome is the expected pass/fail.
• The transcript role/content matches the user message that was fed in.

Why this is a follow-up, not a blocker

The current e2e tests do catch the most important regression — "Custom Engine doesn't run at all" — but the safety net is shallow.

Origin

Raised during the self code-review of feat/custom-engine-local (CI/tests finding #3).