fix: claude code review for forked branches

[priyansh4320]

Why are these changes needed?

The current Claude Code Review GitHub Actions workflow fails when reviewing pull requests from forked repositories with a "file not found" error. This prevents external contributors' code from being automatically reviewed, creating an inconsistent review experience.
Example failing PR: #2097
Root causes:
The workflow uses pull_request trigger, which doesn't provide secrets access to forked PRs
After checking out the base repository, the workflow never switches to the PR branch
Claude attempts to review PR files while still on the main branch, resulting in "file not found" errors

Related issue number

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/docs/contributor-guide/documentation/ to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[joggrbot]

📝 Documentation Analysis

All docs are up to date! 🎉

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✅ Latest commit analyzed: 9a09cf7 | Powered by Joggr

[marklysze]

Hi @priyansh4320

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 41 files with indirect coverage changes

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