GitHub Action: annotate the PR with an eval diff summary

[abhishekmanyam]

Summary

Have the GitHub Action post an eval summary back onto the PR — a sticky comment with the pass-rate delta vs. the baseline, so reviewers see regressions without opening CI logs.

Proposed behavior

After evalcore run ... --baseline <ref>, the action emits a machine-readable summary (we already produce JSON/JUnit reporters) and a composite step upserts a single PR comment:

• overall pass/fail and exit code,
• pass-rate and mean-score delta vs. baseline,
• the newly-regressed cases (names only),
• a collapsible link to the HTML report artifact.

Notes

• Reuse existing reporters — no new output format in the engine unless something's genuinely missing.
• Sticky comment (update in place), not a new comment per run.
• Must degrade gracefully on forks / missing token (skip, don't fail the run).

Good candidate for someone comfortable with GitHub Actions + a bit of the JSON reporter shape.