Add Microsoft 365 external tab for chat context and sync

## Summary
Add a Microsoft 365 area in chat where users can browse OneDrive/SharePoint files, inspect content, and decide whether to sync files into a workspace or use them as live/search-only context.

## User Value
Tracking this as a GitHub issue moves the backlog item out of a private markdown file and gives the team a clear place to prioritize, design, implement, and validate the work.

## Proposed Behavior
This should be split after design into browsing, content preview, sync-to-workspace, and chat-context selection. Permission boundaries must follow the signed-in user and workspace scope.

## Acceptance Criteria
- [ ] The requested behavior is designed or implemented for the relevant SimpleChat surfaces.
- [ ] Permission, configuration, and existing-feature interactions are accounted for.
- [ ] Tests, documentation, or validation notes are added as appropriate.

## Notes
- Source backlog entry: Microsoft 365 External Tab And Default Chat Experience
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Extension / Research
- Current anchors: Microsoft Graph plugin/actions, File Sync, workspace document selectors, chat external tabs
- Proposed roadmap priority: P2
- Proposed roadmap size: L