ci: show when the report artifact expires in the PR comment

[nucliweb]

Summary

The size-regression comment links the HTML report artifact, which GitHub deletes after the repository's retention period (90 days here), so an old PR's link fails with no explanation.

• The comment now reads: "Available until YYYY-MM-DD, when GitHub deletes the artifact."
• The date is the artifact's real expires_at from the API, not a computed 90 days, so it follows the repository's retention setting.
• Adds actions: read so the token can read the artifact.
• If the lookup fails, the comment is still posted without the date and the run logs a warning.

Test plan

• actionlint passes
• The comment script, run against a stubbed GitHub client: with a successful lookup the line ends with the date; with a failing lookup the comment still posts without it and logs a warning
• The real API returns expires_at for an existing report artifact
• This PR's own gate run posts the comment with the expiry date (run 37066920914)

[github-actions]

Codec comparison ✅ no size regression

📥 Download the codec report (unzip and open report.html locally). Available until 2026-12-31, when GitHub deletes the artifact.

Size check

Size regression check (tolerance 2.0%)

  ok         kodim04.png  JPEG XL  base 120534 → 120534  (+0.00%)
  ok         kodim04.png  jpegli  base 130656 → 130656  (+0.00%)
  improved   kodim04.png  HEIC  base 141774 → 141631  (-0.10%)
  ok         kodim04.png  AVIF  base 150014 → 150019  (+0.00%)
  ok         kodim04.png  WebP  base 173748 → 173748  (+0.00%)
  ok         kodim04.png  mozjpeg  base 201372 → 201372  (+0.00%)
  ok         kodim19.png  JPEG XL  base 136558 → 136558  (+0.00%)
  ok         kodim19.png  jpegli  base 147032 → 147032  (+0.00%)
  improved   kodim19.png  AVIF  base 148631 → 148498  (-0.09%)
  ok         kodim19.png  HEIC  base 155171 → 155322  (+0.10%)
  ok         kodim19.png  WebP  base 173918 → 173918  (+0.00%)
  ok         kodim19.png  mozjpeg  base 213006 → 213006  (+0.00%)
  ok         kodim23.png  JPEG XL  base 74113 → 74113  (+0.00%)
  ok         kodim23.png  jpegli  base 97039 → 97039  (+0.00%)
  ok         kodim23.png  AVIF  base 111100 → 111556  (+0.41%)
  ok         kodim23.png  mozjpeg  base 130349 → 130349  (+0.00%)
  ok         kodim23.png  WebP  base 140374 → 140374  (+0.00%)
  ok         kodim23.png  HEIC  base 157392 → 157442  (+0.03%)

OK: no size regressions.