fix(client): keep artifact cards inside agent replies

[auspham]

Summary

Scope comment rail card styles to direct rail children so an artifact element using the generic .card class remains inline inside the cloned agent answer instead of becoming a second fixed rail card.

Extend the browser regression fixture to cover rich answer content containing .card and assert that the outer comment card remains fixed while the nested artifact card remains in normal flow.

Type of change

• Bug fix
• New feature
• Documentation
• Refactor or tooling
• Other:

How was this tested?

The updated browser regression failed before the selector fix with the nested artifact card computed as position: fixed, then passed with it computed as position: static. The exact reported live review card was also checked against the locally synced client.

• python -m pytest tests/python
• ruff check skills/htmlit tests/python
• npm test (browser client)

Checklist

• I added or updated tests for this change (a bug fix has a test that fails before the fix).
• I did not add any runtime dependency to the skill (standard library only).
• I did not commit vendored assets (Mermaid, highlight.js, Idiomorph).
• Commits follow Conventional Commits and are signed off (git commit -s).