chore: prune shipped diff-rules

[github-actions]

Automated diff-rule pruning

prune-diff-rules.yml replayed the stored prod captures against prod and
found the rules below are in the "fix has shipped" state: prod now serves
the new behaviour while the frozen captures still carry the old one, so each
rule is actively suppressing a real prod-vs-capture diff. They have done their
job and can be removed.

Removed:

• scripts/diff-rules/2026-05-promote-zizmor-planned-rules.mjs (id promote-zizmor-planned-rules) — suppressing 19 capture diff(s)
• scripts/diff-rules/2026-06-expression-syntax-stray-brace.mjs (id expression-syntax-stray-brace-fix) — suppressing 1 capture diff(s)

After this merges, the next release: published triggers
rebaseline-captures.yml, which overwrites the stored captures with the
current prod responses and clears the diffs for good.

The dark-launch replay check is expected to go red for these captures
until that rebaseline runs — they still carry the pre-fix findings the rule
was suppressing. That red is the signal the rule was still doing real work,
not a regression.