feat(core): add OSC 8 hyperlink metadata to markdown links

[cevr]

Summary

• Set link: { url } on TextChunks for markdown link and image tokens, enabling OSC 8 terminal hyperlinks
• Thread optional link parameter through createChunk and renderInlineTokenWithStyle — backwards-compatible, no new methods
• All link/image chunk text (label, URL, punctuation) carries the href so the entire rendered span is clickable

The rendering infrastructure (textBufferSetStyledText → linkAlloc → zig \x1b]8;;URL\x1b\\) was fully wired. The only missing piece was Markdown.ts never populating chunk.link.

Test plan

• Link chunks carry link.url with conceal=true
• Link chunks carry link.url with conceal=false
• Image chunks carry link.url
• Non-link text has no link metadata
• Manual: verify Cmd+click opens URLs in iTerm2/Kitty/WezTerm

🤖 Generated with Claude Code

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commit: 03c9ec5