Add ESLint, checked JS, and CI verification

[MarsLuay]

Summary

• Add npm run lint, npm run typecheck, and combined npm run check with ESLint correctness rules, checked JavaScript/JSDoc types, and a lockfile-backed CI workflow.
• Cover queue/job state, durable snapshots (QUEUE_DURABLE_STATE_KEY), runtime messages, queue settings, and the provider adapter contract without converting the extension to TypeScript or adding a bundler.
• Fix real lint/type issues (duplicate refreshTargetTabs, duplicate getComposerFromEventTarget stub, promise executor returns) and document the commands plus suppression policy in CONTRIBUTING.md.

Closes nothing: parent owns merge, verification, and issue #28 closure.

Testing

• Ran local tests or lint commands
• Verified the affected workflow manually

Commands:

• npm test — pass (127 tests)
• npm run lint — pass
• npm run typecheck — pass
• npm run check — pass
• Injected definitelyUndefinedIdentifierForLint — npm run lint failed with no-undef
• Injected QueueJob.tabId: string — npm run typecheck failed with TS2322

Checklist

• Updated documentation when behavior changed
• Followed the code of conduct and contributing guidelines

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