feat: render GitHub issue/PR links GitHub-style with state + title

[16bit-ykiko]

Summary

Bare GitHub issue/PR URLs in the docs now render the way GitHub renders a pasted reference, instead of as a plain link:

• a state-colored octicon leads the link (open = green, closed/merged = purple, closed-PR = red, not-planned = gray; PRs get the git-pull-request glyph);
• the issue/PR title is shown inline (backtick spans become inline <code>);
• a muted owner/repo#number trails the title.

This replaces the previous hand-maintained `open`/`closed` markers in footnotes — authors now just drop a bare GitHub URL and the state + title render automatically.

How it works

• A markdown-it plugin (.vitepress/github-links.ts) rewrites matching issue/PR links at render time and is wired into config.mts.
• Issue/PR state + title are resolved at build time and cached in .vitepress/github-links.json, so the published site is a static snapshot with no client-side API calls.
• The cache refreshes live only when GITHUB_TOKEN/GH_TOKEN is present (CI) or DOCS_REFRESH_GH=1 is set; otherwise local builds stay fully offline and deterministic (no rate limits).
• Explicit markdown links (e.g. changelog [#460](url)) keep the author's text and only gain the leading state icon.

Test plan

• npm run build succeeds (verified locally).
• Footnote-heavy feature pages render [icon] Title owner/repo#number inline (verified via headless screenshot of features/completion).
• Backtick spans in titles render as inline code (e.g. clangd#2626 → import, module, CompletionItemKind.Keyword).
• Icon size/alignment matches surrounding text.
• No live API calls on a tokenless build (offline cache used).