kit_doctor's drift self-check makes mutation testing report false kills

[topij]

The problem

test_kit_doctor.py::test_kit_repo_self_check_is_clean rehashes every kit-owned
file against kit-manifest.json. So any byte changed in an engine fails it —
including a deliberate mutation. A mutation-testing run therefore reports
"killed" for every mutant while nothing behavioural caught anything.

Reproduce in ten seconds — disable _flat's truncation entirely (a pure
behaviour change no drift check should care about):

def _flat(text: object, n: int = 999999) -> str:   # was 120

FAILED scripts/tests/test_kit_doctor.py::test_kit_repo_self_check_is_clean
1 failed, 316 passed

Looks killed. Nothing behavioural touched it.

Why this is worse than a papercut

It is retroactive. docs/agentic-dev-kit/fallback-review-panel.md makes
mutation testing contract item 5, and "N mutants died" was used across #25, #28,
#29 and #31 as evidence that a property was pinned — and, more consequentially,
as a reason to stop reviewing. Any of those runs where the reviewer did not
exclude this test is worthless.

Measured on #31: one lens's first pass reported 17/17 killed. Re-run with the
drift test excluded: 7 had survived — including the exact guard the commit
message under review was advertising as fixed.

A false-negative testing tool is worse than no tool, because it is used to
justify confidence.

Proposed fix

Make the exclusion a documented one-liner rather than folklore:

@pytest.mark.driftcheck
def test_kit_repo_self_check_is_clean(): ...

• register the marker in pytest config
• uv run pytest -m 'not driftcheck' becomes the documented mutation-testing
  invocation
• reference it from fallback-review-panel.md contract item 5 (which currently
  warns in prose, helping the panel but not anyone mutating by hand) and from
  wherever /code-review describes mutation testing

Alternative considered: regenerate the manifest after each mutation. Works,
but it is per-mutant bookkeeping every reviewer must remember, and forgetting it
fails silently in the confident direction.

Scope note

This is not specific to the panel — it applies to any agent or human
mutation-testing a kit-owned engine, which the kit now actively asks for.