Design Office file modification workspace

[paullizer]

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: generate word, ppt, excel, and csv as outputs #482 covered output generation; this issue focuses on modifying existing Office/CSV files alongside an agent.