ci: redirect new issues and pull requests to Apache

[zjw1111]

Purpose

Linked issue: N/A.

New issues and pull requests opened in alibaba/paimon-cpp should direct contributors to the project's new Apache home. Add an automatic comment explaining the migration and linking to apache/paimon-cpp, with direct links for opening an issue or submitting a pull request.

The workflow runs only in alibaba/paimon-cpp and only for newly opened issues and PRs. It checks for an existing bot notice to avoid duplicate comments on reruns. Fork PRs are handled through pull_request_target; the workflow does not check out or execute PR code. The action is pinned to a commit and token permissions are limited to issues and pull requests.

Tests

• Passed: pre-commit run --files .github/workflows/migration_notice.yaml.
• Passed: git diff --check and git diff --cached --check.
• Passed: local Python/Node validation of the workflow triggers and four mocked comment scenarios: no existing comment, existing bot notice, a contributor comment containing the marker, and a comment with missing user/body.
• C++ build and unit tests were not run because this change only adds a GitHub Actions notification workflow.
• Live GitHub event delivery has not been tested; the workflow becomes active after merging into the default branch.

API and Format

No public API, storage format, or protocol changes.

Documentation

The automatic reply communicates the existing migration notice and links contributors to the Apache repository. No separate documentation changes are needed.

Generative AI tooling

Generated-by: OpenAI Codex (GPT-6).

[lxy-9602]

+1