fix(ci): un-vacuum the mutants-core gate; dedupe the v0.33.0 changelog section

[avrabe]

Two findings from the v0.33.0 pre-tag clean-room audit (9/9 claims passed; these were flagged alongside). Neither blocks the tag — the first is nightly + continue-on-error, the second cosmetic — but the first is the exact defect class this release exists to remove, so it should not outlive the release that introduced it.

1. mutants-core was made unconditionally vacuous — by my own REQ-288 follow-up

The empty-diff short-circuit (if [ ! -s pr.diff ]; then … exit 0) belongs to mutants-cli, which computes pr.diff. My patch applied it to every matching check step, and mutants-core has no step that writes pr.diff — different job, different runner label, and actions/checkout cleans untracked files anyway. So the guard always fired and the job never evaluated outcomes.json or missed.txt for rivet-core. The in-file comment claiming "honest numbers surface without blocking" was false as written.

Root cause worth naming plainly: a string replace without a count. The fix for a vacuous gate introduced a vacuous gate one job over — inside the release whose thesis is that a gate which cannot fail is not a gate.

Guard removed from mutants-core; mutants-cli keeps it (correct there). Verified: mutants-core now has 0 pr.diff references, mutants-cli retains 5.

2. Duplicated ### Fixed in the [0.33.0] changelog

The section carried two ### Fixed subheads and documented REQ-290 twice (my summary plus #776's fuller entry). Collapsed to one, keeping the detailed entry. A duplicated section in a release about honest reporting is worth fixing before the tag rather than after.

Now: one ### Fixed, REQ-290 mentioned once, all of REQ-288/289/290/291 present.

yamllint clean · docs check 0 violations.

Refs: REQ-288

[codecov]

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