Add news release actions

[jezdez]

Description

Adds conda-owned release-notes actions for the fragment-first news flow:

• check-news validates sectioned conda news snippets on pull requests without writing files and honors the no-news label.
• prepare-release runs only from a trusted workflow_run release-branch push context, aggregates existing snippets into CHANGELOG.md, deletes consumed snippets, and opens or updates a release-notes PR.
• Shared parsing lives in news_common so PR-time validation and release-time rendering use the same section rules.

Refs conda/infrastructure#556 and conda/infrastructure#1378

Related PRs:

• Pilot news release actions conda#16194
• Update release notes process infrastructure#1356

Validation:

• uv run --with ruff ruff check check-news prepare-release news_common
• uv run --with pytest --with pytest-cov --with pytest-mock python -m pytest check-news prepare-release
• Full suite currently has 111 passing tests and 2 unrelated existing Rich ANSI assertion failures in template-files/test_template_files.py.

[dholth]

Does it continue updating a release PR as PR's are merged into main?

[danyeaw]

Hey @jezdez, I exercised this on https://github.com/conda-sandbox/example. The happy path looks great and will be a big improvement.

One edge case: after the release-notes PR is merged into YY.MM.x but before YY.MM.MICRO is tagged, a later news fragment makes prepare-release fail with Changelog already contains an entry for … (example: https://github.com/conda-sandbox/example/actions/runs/31508633466).

So I think prepare-release should treat an existing changelog heading as still open if that version isn’t tagged yet and amend that section.

I also considered an alternative which would be that PRs to a release branch could change the changelog directly instead, but keeping news fragments feels cleaner.

[jezdez]

Thanks for exercising this, @danyeaw. prepare-release now updates the existing changelog section while the inferred version remains untagged, preserving the original heading and surrounding changelog content. I also updated the token example, removed fragment-format, and documented the exempt-authors replacement behavior.

[danyeaw]

Thanks @jezdez — re-checked against my feedback. Docs/token/fragment-format/exempt-authors look good, and the changelog amend path matches the post-merge pre-tag case.

Re-validated on conda-sandbox/example with a fresh 26.9.x line pinned to c76832d:

• First release-notes PR: conda-sandbox/example#9
• Post-merge second fragment amend succeeded: https://github.com/conda-sandbox/example/actions/runs/31600748458 → conda-sandbox/example#10

LGTM.

[danyeaw]

Hi @jezdez, thanks for the updates, these look great. I also tested the post-release merging more news with conda-sandbox/example#9 and conda-sandbox/example#10, it is working great.