Run Roadmap automation when pull requests merge

[YanxuanLiu]

Summary

• keep adding issues and pull requests to the NVIDIA Project when they are opened
• also run the shared action when a pull request is closed and merged
• skip pull requests that are closed without merging
• temporarily use spark-rapids-common/add-to-project@auto-roadmap-from-pom for integration testing

Behavior

The shared action adds the PR to the Project on opened, but only derives and sets Roadmap on the merged closed event. If Roadmap was assigned manually before merge, it is preserved.

Testing note

This remains a draft integration PR. pull_request_target uses the workflow from the base repository's default branch, so this PR's new closed trigger will not execute until the workflow change exists on the default branch. Switch the action reference back to @main after NVIDIA/spark-rapids-common#59 is merged.

Validation

• YAML parsed successfully
• git diff --check
• event-condition review: issue opened and PR opened run; merged PR runs; closed-unmerged PR is skipped

Common action PR: NVIDIA/spark-rapids-common#59