Fixes inquid/github-chat#1.

Implements the extension-side chat UI requested by the sponsored GitHub Chat issue.

Changes:

• Adds a separate GitHub-only content script that runs on https://github.com/*.
• Adds a lower-right Chat toggle that opens and closes a chat panel.
• Scopes messages by GitHub repo/page and stores them locally in the browser.
• Keeps the existing stories dashboard content script and styles scoped to the original dashboard matches.

Verification:

• node --check content-script.js
• node --check github-chat.js
• parsed manifest.json successfully