docs: migrate PR review action references to OpenHands/extensions plugin

[srxly888-creator]

Summary

This updates docs references from the deprecated SDK action path to the current extensions plugin path for PR review.

Changes

• openhands/usage/use-cases/code-review.mdx
  • Replace OpenHands/software-agent-sdk/.github/actions/pr-review@main with OpenHands/extensions/plugins/pr-review@main in workflow snippets
  • Update related resource links to OpenHands/extensions/plugins/pr-review
• openhands/usage/essential-guidelines/sdlc-integration.mdx
  • Replace inline reference OpenHands/software-agent-sdk/.github/actions/pr-review with OpenHands/extensions/plugins/pr-review

Why

The PR review plugin has been migrated to the extensions repository, so docs should point to the maintained action path.

Related context

• 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 #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 extensions#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.