fix(code): render markdown italics in inherited font

[thmsobrmlr]

Summary

• Replaces Radix UI Themes' <Em> with a plain <em> in the markdown renderer so italicized text inherits the surrounding font instead of switching to Radix's serif italic, which made italics look noticeably larger/different.

Test plan

• Render markdown containing italics (*word* / _word_) in MarkdownRenderer and confirm italic text matches the size and family of surrounding text.
• Verify markdown still renders correctly across affected surfaces: agent messages, user messages, queued messages, signal cards, skill detail panel, PR comment threads.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(code): render markdown italics in in..." | Re-trigger Greptile}