Add Copilot coding agent instructions

[Copilot]

Adds .github/copilot-instructions.md to provide repository context for GitHub Copilot coding agents.

Contents:

• Repository overview — Chrome MV3 extension stack (vanilla JS, Web Crypto, IndexedDB), no build step
• Build/test commands — npm install, npm test (Vitest), extension loading workflow, E2E test setup
• Architecture — Directory structure, component breakdown (content scripts, background worker, storage engine, GitHub GraphQL sync)
• Code conventions — ES6+ vanilla JS requirement (no frameworks), security patterns (AES-GCM token encryption, sender validation), error handling
• Known issues — DOM selector fragility across AI platforms, Chrome storage quota (~5MB), existing test failures in storage.test.js (mock configuration issues, not functional bugs)

Reduces agent exploration time by documenting: no transpilation required, batch sync uses GraphQL not REST, content scripts use MutationObserver for conversation capture, files stored as {platform}/{date}/{id}.json.gz.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• gh.io
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes✨ Set up Copilot instructions #2

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.