Add GitHub Chat extension widget

[namnv080808]

This adds an MVP GitHub Chat widget to the existing GitHub Stories Chrome extension.

Changes:

• Expands the content script match pattern from the GitHub dashboard/org pages to https://github.com/*.
• Adds github-chat.js, which injects a fixed Chat button in the bottom-right corner.
• Adds a toggleable chat panel with Escape/close-button support.
• Loads the existing GitHub Chat web app in an iframe.
• Passes the current repository and page URL as query params when available.
• Adds responsive styling in github-chat.css.

Manual checks:

• manifest.json parses successfully.
• Content script now matches https://github.com/*.
• New JS/CSS files are included in the manifest.
• Patch applies cleanly with git apply --check.