test(release): Tighten test_release_helpers.sh assertions and cover registry-baseline's jq guard

Summary

Advisory findings from #455's final review round, deferred rather than folded — the PR was already mergeable and no blocking round was open (rules/review-severity.md Split Reading From Acting, rules/boy-scout.md Fold Into a Round Already in Flight).

Both in skills/release/tests/test_release_helpers.sh:

1. Regex dots in literal assertions. grep -q "carries no .version" and grep -q "carries no .current" treat . as a wildcard, so the assertion still matches if the diagnostic stops naming the literal field. Use grep -qF.
2. registry-baseline.sh's jq guard is uncovered. The suite exercises the absent-jq path for confirm-tessl-landed.sh only, though registry-baseline.sh has the same guard. Add the mirrored case using the same PATH-shadowing approach.

Neither changes shipped behavior — both make the suite stricter about behavior that is already correct.

Test plan

• Both assertions use grep -qF and still pass
• Mutating either diagnostic to drop the literal field name fails the suite
• A jq-less PATH makes registry-baseline.sh exit 2 naming jq

Start in skills/release/tests/test_release_helpers.sh and inspect the existing assertions and PATH-shadowing case for confirm-tessl-landed.sh. Use the issue's test plan as the completion checklist: literal-field assertions must remain strict, the registry-baseline.sh jq-less case must be covered, and the suite must pass.