Add GitHub Chat as a Chrome extension

[martrend33]

Summary

Adds a Manifest V3 Chrome extension that injects a GitHub Chat toggle on github.com pages. The toggle appears in the lower-right corner, opens a compact iframe panel when the chat endpoint allows embedding, and falls back to an Open Chat action for endpoints that block iframes.

What changed

• Adds a GitHub-only extension manifest.
• Adds a content script to inject the toggle, iframe panel, and current-page context.
• Adds fixed lower-right UI styles.
• Adds an options page so maintainers can configure the deployed chat endpoint without editing source.
• Adds a fallback for github-chat.com, which currently sends X-Frame-Options: DENY.

Verification

• Parsed manifest.json successfully from the forked GitHub copy.
• Ran node --check on content-script.js from the forked GitHub copy.
• Ran node --check on options.js from the forked GitHub copy.

Refs inquid/github-chat#1