test(release): assert the release-title contract, not which file restates it (#3955)

`test_agent_and_human_guidance_explain_the_release_title_contract` required
both AGENTS.md and CONTRIBUTING.md to contain "fix(cua-driver):",
"feat(lume):", "no-release" and "squash".

#3927 ("docs: deduplicate repository agent guidance") deliberately removed
that restatement from AGENTS.md and replaced it with a link to
CONTRIBUTING.md, which is now the canonical copy and still carries all four
strings. The test then failed on `main` itself -- and because CI tests the
merge result, on EVERY open pull request, regardless of content. Two of the
three red checks on #3883 were this, not that PR.

Restoring the literals to AGENTS.md would undo the deduplication and bring
back the two-copies-that-drift problem #3927 was written to fix. The
contract did not change; its home did. So assert what actually matters: the
contract is documented in the canonical file, and an agent reading AGENTS.md
is pointed at it.

Verified three ways rather than assumed:
  - the original test fails on main (the bug, reproduced);
  - the new test passes on main;
  - the new test still FAILS when CONTRIBUTING.md is mutated to drop
    "fix(cua-driver):", so it is not weakened into a tautology.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>