Design Office file modification workspace

## Summary
Add a UI where users can modify Word, PowerPoint, Excel, and CSV files alongside an agent.

## 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 is larger than export. Define whether SimpleChat edits binary Office files directly, creates new revisions, uses external services, or delegates to generated artifact workflows.

## 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: Office File Modification Workspace
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: New / Research
- Current anchors: document upload/extraction, message export, generated artifacts, SimpleChat native document operations
- Proposed roadmap priority: P2
- Proposed roadmap size: XL
- Related: #482 covered output generation; this issue focuses on modifying existing Office/CSV files alongside an agent.