Rename validate-changelog to changenotes

[mbg]

Follow-up to #4128. As @mario-campos noted there, the script will have functions other than validation in the future. This PR renames it to reflect that it offers functionality related to change notes. We already have scripts that work directly on the overall changelog (changelog.ts and (bundle|prepare|rollback)-changelog.ts), which is distinct from the new change notes files, so the changenote.mts name makes sense.

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?

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?

• Other - Please provide details.

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

🟢 Approval recommended

The focused rename is consistent and no stale references remain.

Review tier: Balanced
Findings: None

What changed in this PR

Renames the change-note utility and aligns its CLI help text with the new name.

Changes:

• Updates the usage message from validate-changelog.mts to changenotes.mts.

File	Description
pr-checks/​changenotes.mts	Updates the displayed CLI command name.

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