Artifact panel: AI-native actions (Ask Claude about this file, quick actions)

[itsdestin]

From the artifact viewer panel (SessionDrawer), let the user act on the open file via Claude:

• "Ask Claude about this file" — drop a templated prompt referencing the artifact into the active session.
• Quick actions for docs: Summarize, Improve/polish, Make shorter/longer, Change tone, Translate.
• Select text in the viewer → "Ask Claude about this part".

This is the differentiating feature vs a plain file viewer — editing/asking by conversation from the file itself. Deferred from the 2026-06 artifact-panel UX pass.