Add SimpleChat plugin file inventory and tagging operations

@paullizer is already working on this.

Since Jun 26, 2026.

## Summary
Extend the SimpleChat native plugin so agents can tag files, count files, summarize file types, count tags, and report files per tag in authorized workspaces.

## 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
Add explicit capability toggles. Enforce the same personal/group workspace permissions as existing document APIs. Return structured counts suitable for charting.

## 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: SimpleChat Plugin File Operations
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Extension
- Current anchors: `application/single_app/functions_simplechat_operations.py`, `application/single_app/semantic_kernel_plugins/simplechat_plugin.py`
- Proposed roadmap priority: P2
- Proposed roadmap size: M