Add changenotes.mts assemble command to move changenotes to CHANGELOG.md

[mario-campos]

This PR extends the pr-checks/changenotes.mts functionality by adding an assemble command that will be used to "flush" (or empty the contents of unreleased-change-notes) to the [UNRELEASED] section of CHANGELOG.md.

This PR also replaces the use of constants 0 and 1 for process exit codes with the ExitCode type from @actions/core.

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).

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

Empty or invalid inputs can mutate the changelog, and deletion failures can leave a partially completed flush.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 3

Open (3)

• Preserve the placeholder when there are no notes · New
• Validate changenotes before deleting them · New
• Avoid partial flushes when deletion fails · New

What changed in this PR

Adds a development command to move unreleased changenotes into CHANGELOG.md.

Changes:

• Adds the changenotes.mts flush command.
• Adds changelog insertion helpers and unit tests.
• Normalizes the default no-changes placeholder.

File	Description
pr-checks/​changenotes.mts	Implements changenote flushing and deletion.
pr-checks/​changelog.ts	Adds unreleased-section insertion logic.
pr-checks/​changelog.test.ts	Tests header parsing and insertion behavior.
pr-checks/​bundle-changelog.test.ts	Updates assertions for the normalized placeholder.

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[mbg]

Thanks for preparing this! I think functionally this looks fine, but as always, I have had a few comments and suggestions on the details of this initial implementation.

[mbg]

Thanks for addressing the feedback! This continues to look good in terms of functionality. I have a few more comments with minor suggestions and there was an inconsistency or two I spotted, but I don't think there's anything major that's blocking this, so I am happy to approve.

[mbg]

Thanks!