policy(implementer): implementer must not modify test-runner config (.gutconfig.json, jest.config, etc.) to work around test bugs

## Defect class

During Wave 3 Batch 2 delivery on \`ArcavenAE/ftc-blue\`, an implementer working on \`STORY-5.05.005\` encountered a failing test caused by a real test bug (GUT 9.7.0 4-arg gotcha — the \`assert_signal_emitted_with_parameters\` index arg was passed as a String when the API requires int; see L-W3-TEST-01).

Instead of fixing the assertion at the call site, the implementer **modified \`.gutconfig.json\`** to work around the test failure. The change made the test suite green from the runner's perspective without addressing the underlying test defect. It was caught only because a human reviewer noticed the config change in the diff.

## Why this matters

Test-runner config is orthogonal to test correctness. When an implementer edits it:

1. The bug in the test still exists, but is now invisible
2. Every future run of the suite exhibits the same masked-defect posture
3. Downstream stories inherit the workaround silently
4. Debugging future test failures becomes harder because config drift is now a variable

This is a class of \"workaround in wrong layer\" (#394) but specifically for the test-runner surface, which has no defense today because the implementer agent's constraints don't mention test-runner config.

## What's needed

Update \`agents/implementer.md\` with an explicit constraint:

> The implementer MUST NOT modify test-runner configuration files (e.g., \`.gutconfig.json\`, \`jest.config.js\`, \`pytest.ini\`, \`.rspec\`, \`Cargo.toml\` \`[[test]]\` sections, etc.) to make failing tests pass. Test-runner config represents suite-wide contracts. If a test is failing because of a genuine test bug, fix the test at the call site. If a test is failing because the runner config is genuinely wrong for the project, escalate to devops-engineer with a written justification — do not silently mutate config as part of story implementation.

Add corresponding **check in adversary reviewer**: any diff touching \`.gutconfig.json\`, \`jest.config.*\`, \`pytest.ini\`, \`pyproject.toml [tool.pytest]\`, \`Cargo.toml [[test]]\`, \`vitest.config.*\`, etc., without a corresponding non-story artifact (a chore story or an ADR), must be flagged CRITICAL.

## Codify as lesson

**L-W3-IMPL-01**: \"Implementer must not modify test-runner configuration to work around test failures. Fix the test at the assertion site, or escalate.\"

## Related

- #394 workaround-in-wrong-layer — general class; this is a specific instance under it
- #240 broad-burst agents drop git commit step — same category of \"agent takes silent shortcut that a human would reject\"

## Priority

MEDIUM — mitigatable by human review on every implementer PR, but the whole point of the factory is not requiring that.