ci: improve codex prompt for backport and ci fix

[jackye1995]

1. allow backporting multiple PRs in the same run
2. use a more meaningful PR title for CI fixes

[github-actions]

PR Review

Summary: This PR improves the Codex prompts for the backport and CI fix workflows. The changes are straightforward and low-risk.

Changes Overview

1. Backport workflow: Extended to support comma-separated list of PR URLs for batch backporting
2. CI fix workflow: Improved PR title to be more descriptive instead of generic

Assessment

No P0/P1 issues found. These are prompt-only changes to internal CI workflows with no impact on the core codebase.

The changes look reasonable:

• The multi-PR backport logic is clearly documented in the prompt
• The diff command change from HEAD~1 to ${RELEASE_BRANCH}...HEAD is correct for multi-commit scenarios
• The CI fix PR title change is a minor UX improvement

LGTM ✅

[wjones127]

Exciting!