Agentic UI: Allow for editing chat titles, and adds new description metadata

[shaunandrews]

Related issues

• None.

How AI was used in this PR

AI helped split this change out of the larger interface-improvements branch, then verify the diff stayed focused on session metadata and editable chat titles.

Proposed Changes

• Gives Studio chats clearer saved identity by generating session titles/descriptions and letting users edit chat titles/details in the classic UI.
• Keeps the main interface-improvements PR focused on the classic shell by moving this metadata behavior into a stacked PR.

Testing Instructions

• npm run typecheck
• npm test -- apps/studio/src/modules/ai-agent/tests/session-metadata.test.ts tools/common/lib/tests/shared-config.test.ts apps/ui/src/components/site-list/index.test.tsx apps/ui/src/components/site-overview-view/index.test.tsx

Pre-merge Checklist

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