Incorporate change-notes into CHANGELOG.md at release

[mario-campos]

This PR automates the effective execution of pr-checks/changenotes.mts assemble into the update-release-branch process by refactoring the code to be importable, and then importing it into pr-checks/update-release-branch.ts.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Workflow types:

• N/A

Products:

• N/A

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).
• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).
• Other - Manual testing

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

How will you know if something goes wrong after this change is released?

• N/A

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Copilot review overview

🟡 Changes recommended

Changenote deletions are not committed and can occur before the changelog is successfully written.

Review effort: Balanced
Findings: 1

Open (1)

• Delete changenotes only after changelog write succeeds · New

What changed in this PR

Automates incorporating unreleased changenotes into the changelog during release preparation.

Changes:

• Extracts reusable changenote transfer logic.
• Integrates changenote assembly into release branch preparation.
• Updates ESM entry-point detection.

File	Description
pr-checks/​changenotes.mts	Adds reusable changenote assembly and cleanup.
pr-checks/​update-release-branch.mts	Runs assembly when updating the release changelog.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The release workflow integration is focused and correctly stages both the populated changelog and removed changenotes.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Delete changenotes only after changelog write succeeds

[mbg]

This basically LGTM. I'd say that we should hold off from merging this until the current CLI release has wrapped up though (including the CodeQL Action release that's part of that process) so that, if there is a problem with this, we don't accidentally throw a spanner into that process -- especially since we don't currently have any change notes.

Two things I'd like to see on top of what's here:

1. A workflow (or check in an existing one) that runs for ordinary PRs (not release PRs) and which complains (but doesn't necessarily fail) if the PR changes CHANGELOG.md directly. That can be an independent PR from this one.
2. It would be good to improve our test coverage here for this. I know that changenotes.ts has its own ones that test the functions there, but it would be good to have something more end-to-end. Let's chat about how to best approach that.

[mbg]

Odd