Reduce Studio present artifacts to site previews

[shaunandrews]

Related issues

• Follow-up to Remove ui-desks interface #3852

How AI was used in this PR

Codex made the scoped cleanup based on follow-up review feedback from #3852, checked whether the old widget artifact surface still had active consumers, and ran the validation commands listed below.

Proposed Changes

• Simplifies Studio Code's visual artifact surface to live site previews, matching the only artifact experience that still renders after Desks removal.
• Replaces the generic studio_present widget tool with show_site_preview, which only opens the current local site's live preview at a local path.
• Removes the Desk-era widget manifest and generic artifact payload shape, so the runtime emits a direct sitePreview artifact instead of a list of widget descriptors.
• Keeps screenshot capture focused on agent visual analysis, with eval coverage verifying returned image results directly instead of media-widget temp-file payloads.
• Preserves transcript compatibility by continuing to hide old studio_present tool rows in existing conversations.

Testing Instructions

• npx eslint --fix apps/cli/ai/chat-artifacts.ts apps/cli/ai/system-prompt.ts apps/cli/ai/tests/system-prompt.test.ts apps/cli/ai/tests/tools.test.ts apps/cli/ai/tools/create-site.ts apps/cli/ai/tools/define-tool.ts apps/cli/ai/tools/index.ts apps/cli/ai/tools/show-site-preview.ts apps/studio/src/components/studio-code-session/conversation/index.tsx apps/studio/src/modules/ai-agent/run-manager.ts apps/ui/src/hooks/use-session-commands.ts apps/ui/src/ui-classic/components/session-view/conversation/index.tsx tools/common/ai/chat-artifacts.ts
• npm test -- apps/cli/ai/tests apps/studio/src/components/studio-code-session/conversation/index.test.tsx
• npm run typecheck
• git diff --check

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?