fix: open conflict sync PRs and comment [ED-25668]

[davseve]

Summary

• Cherry-pick squash commits when syncing release branches, so binary files such as Playwright snapshots apply instead of failing with cannot apply binary patch without full index line.
• When a sync conflicts, commit the conflicted tree and open the draft PR. Text files keep conflict markers. Binary files use the incoming version.
• Skip opening a PR when that commit would be empty, which is what made GitHub reject the draft with No commits between.
• Comment on the original PR when a downstream sync fails, including the git or GitHub error and the draft link when one was opened.

Test plan

• Merge a squash PR into release/stable that changes a PNG snapshot and confirm sync PRs open against release/beta and main.
• Force a text conflict and confirm the draft PR contains conflict markers plus a comment on the original PR with the cherry-pick error.
• Confirm a clean sync does not comment on the original PR.

Made with Cursor

✨ PR Description

1. Problem & Context

The current sync process fails silently or provides poor visibility when merge conflicts occur. This update ensures conflict resolution is actionable by creating draft PRs and notifying the original PR author.

2. What Changed (Where)

• actions/sync-release-branches/sync-release-branches.sh: Overhauled conflict handling, added binary conflict resolution, and implemented GitHub PR commenting.
• actions/sync-release-branches/README.md: Updated documentation to reflect new conflict and notification behavior.

3. How It Works

The sync_to_target flow now captures detailed error output from git/GitHub operations. If apply_merged_content fails, create_conflict_pr handles binary files via --theirs and opens a draft PR. Finally, comment_on_original_pr posts the failure details and the draft PR link back to the source PR.

4. Risks

Github API rate limits or token permissions could cause gh pr comment or gh pr create to fail, though these are wrapped in warnings to prevent blocking the entire pipeline.

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