[Dev-iL]

related: #67799

Add the airflow-async-endpoints SKILL to guide and standardize the incremental conversions and new async endpoints for the migrations tracked in #67799. The skill captures lessons from the heartbeat conversion in #67800, including transaction ownership, commit-before-response behavior, synchronous compatibility boundaries, and async test-engine lifetimes. It also covers secrets resolution, contract regression tests, and verification across SQLite, PostgreSQL, and MySQL, with references to the current implementation and driver guidance.

The skill is most useful for the duration of the migration, and especially if additional contributors participate in the effort. Afterwards it can either be removed or be used as a reference for adding more endpoints.

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Generated-by: Codex following the guidelines

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[Dev-iL]

Marked as draft to make room for other PRs