Add eslint for pr-checks

[mbg]

(Based on #3573, which should be merged first.)

Adds an eslint configuration for the pr-checks folder (largely based on the root configuration), updates the .ts files in pr-checks to resolve some linter errors, and adds the linter check to the CI job for pr-checks.

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?

• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).

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

Adds ESLint coverage for the pr-checks TypeScript code and updates CI and scripts so pr-checks code is linted and tested consistently alongside the main repo.

Changes:

• Extend the root ESLint flat config to include pr-checks/**/*.ts using pr-checks/tsconfig.json.
• Refactor pr-checks TypeScript to satisfy linting/type-checking (step typing and string template cleanups).
• Update CI workflows and scripts (caching, installs, and sync-back filename) and introduce npm workspaces wiring.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pr-checks/sync.ts	Adds a minimal Step type and refactors string construction to be lint-friendly.
pr-checks/sync-back.ts	Formatting/import tweaks to satisfy lint rules.
pr-checks/sync-back.test.ts	Updates test text/import path and changes test registration style to satisfy linting.
eslint.config.mjs	Enables linting for pr-checks with the correct TS project and adjusts rules.
.github/workflows/pr-checks.yml	Adjusts caching and install steps; runs pr-checks tests without a local npm ci step.
.github/workflows/rebuild.yml	Updates script filename from sync_back.ts to sync-back.ts.
package.json	Adds npm workspaces configuration to include pr-checks.
package-lock.json	Updates lockfile to reflect workspaces and newly installed workspace deps.

Comments suppressed due to low confidence (1)

pr-checks/sync-back.test.ts:43

• Using async () => { await it(...) } changes the usual node:test pattern (register tests synchronously) and can subtly affect execution/ordering because it() is being treated as an awaitable. Once no-floating-promises is adjusted (e.g. via the ESLint safe-call allowlist), prefer keeping describe synchronous and calling it(...) without await (or using the conventional registration pattern) throughout this file.