Convert sync_back.py to TypeScript

[mbg]

Follow-up (and based on) #3526.

This PR converts sync_back.py and test_sync_back.py to TypeScript equivalents.

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?

• Other - Please provide details.

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]

Pull request overview

This PR adds a TypeScript implementation of the sync_back utility used by PR checks tooling, and updates the rebuild workflow to use the new TS script when syncing Dependabot action-version updates back into the pr-checks sources.

Changes:

• Add pr-checks/sync_back.ts to scan generated workflows and sync action versions back into pr-checks/sync.ts and pr-checks/checks/*.yml.
• Add pr-checks/sync_back.test.ts with Node test coverage for the new TS implementation.
• Update .github/workflows/rebuild.yml to run the new sync_back.ts via tsx during Dependabot rebuilds.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
pr-checks/sync_back.ts	New TS sync-back implementation that updates template/action version references based on generated workflows.
pr-checks/sync_back.test.ts	New Node tests for the TS sync-back implementation.
.github/workflows/rebuild.yml	Switch rebuild workflow sync-back step from Python to npx tsx sync_back.ts.

[esbena]

Despite this being a migration of existing code, I do want a somewhat significant change. Could we take the various regular expression based parsing we have and move that into its own module? A file-internal dictionary is fine.

My main motivations for requesting this are:

• I worry about redundant patterns currently.
• I worry about too-specific patterns, where it is hard to see that they're wrong because they are placed in a very specific coding context.
• They are general parsings that deserve their own very detailed JSDoc descriptions.

[esbena]

Please see the general comments about regex parsing, but as an example of that, this is one of three regex that uses the uses: prefix...