[finding] the conversions-only exit-code cell of `os validate --json --strict` is documented but untested — every fixture raises zero conversions

[claude]

Split out of #11174 / PR #11299, which introduced the behaviour described here. Filed
unassigned, not routed, not graded.

What is unpinned

#11299 makes os validate --json --strict exit non-zero on the same advisory set the
text face fails on. That set is the text face's own warning list, which folds in the
ADR-0087 D2 load-time conversion notices — while the JSON payload carries those notices
separately, under conversions, and its warnings field is the five-way spread without
them.

The reachable consequence, correct and deliberate, is that a config whose only
advisories are conversion notices now exits 1 with:

{ "valid": true, "warnings": [], "conversions": [ ... ] }

That cell is documented — PR #11299's changeset states it outright, and
commands/validate.ts carries the reasoning at the emitJson call — but it is not
tested. The pin added in #11299
(packages/cli/src/commands/validate-json-strict-exit.e2e.test.ts) covers the
advisories-present and no-advisories ends and a run without --strict; both of its
fixtures raise zero conversions, as does every fixture in
packages/cli/test/validate-json-warning-parity.e2e.test.ts, which asserts
conversions: [] explicitly as a declared scope boundary.

So the one cell where the exit code is decided by a collection absent from the
warnings field is the one cell no test exercises. A regression that narrowed the gate
back to the payload's warnings field would restore the original divergence for
conversions-only configs and every existing test would stay green.

Why it was not done in #11299

Judged not cheap enough to ride along, and the seat was asked to say so rather than
stretch: it needs a new fixture (no existing one raises a conversion) plus a full
@objectstack/cli dependency-closure rebuild, i.e. another multi-minute hold on the
container's shared verify lock.

Suggested shape

A third fixture on the existing describe, built from that file's CLEAN_SOURCE plus one
key that trips a live conversion — object-compactLayout-to-highlightFields in
packages/spec/src/conversions/registry.ts looks like the cheapest candidate
(compactLayout: ['title'] on the object). Assert, on one run of
os validate --json --strict: exit non-zero, warnings empty, conversions non-empty.
Confirm first that the key raises no other advisory — unknown-key lints run on the
post-conversion normalized, so it should not, but that is the thing to measure rather
than assume, or the fixture pins the wrong cell.

⚠️ Verify the chosen conversion is still registered when this is picked up; entries
retire on a schedule, and a fixture whose conversion has retired raises nothing and the
test passes vacuously — the failure mode this card is about.

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