docs(pr-review): clarify README migration + custom skill guidance

[srxly888-creator]

Summary

This PR applies a small, targeted README consistency fix for the PR review plugin.

Changes

• File: plugins/pr-review/README.md
• Clarify custom skill example to use a unique skill name (custom-codereview-guide) instead of overriding the default public code-review skill
• Keep /codereview trigger guidance while explicitly framing this as a supplement pattern
• Keep migration section intact, but improve consistency around extensions-era usage
• Expand Security notes to clarify when pull_request_target is appropriate and when lower-trust/comment-only setups should prefer pull_request

Scope

• Minimal, documentation-only change
• No workflow logic changes in this PR

Context

• Related migration discussion: PR reviewer plugin should live in the extensions repo software-agent-sdk#2282

[srxly888-creator]

Hi maintainers — I opened two small, scoped PRs that address the same PR-review migration/clarity issue from the integrator side:

• docs: migrate PR review action references to OpenHands/extensions plugin docs#419: migrates docs from the old OpenHands/software-agent-sdk/.github/actions/pr-review path to OpenHands/extensions/plugins/pr-review
• docs(pr-review): clarify README migration + custom skill guidance #121: updates the plugin README to match current usage and reduce two easy integration footguns:
  • use a uniquely named custom skill example instead of overriding name: code-review
  • clarify when pull_request_target is appropriate vs when safer pull_request is better for low-trust / comment-only smoke-test setups

These are intentionally small and non-invasive, but together they make the migration path much clearer for downstream adopters. Happy to adjust wording or split anything further if that helps review.