Add Workspace Files browser

[PureWeen]

Summary

The SDK provides a Workspace Files API for browsing and managing the session's scratchpad files. PolyPilot does not expose this.

SDK API (session.Rpc.Workspace)

Method	Purpose
ListFilesAsync()	List files in session workspace
ReadFileAsync(path)	Read a workspace file
CreateFileAsync(path, content)	Create a workspace file

Events

• SessionWorkspaceFileChangedEvent — workspace file changed

Proposed Implementation

1. Workspace panel (sidebar or tab) — tree view of session workspace files
2. File viewer — click to view file contents with syntax highlighting
3. File creation — create new workspace files from PolyPilot
4. Real-time updates via SessionWorkspaceFileChangedEvent
5. Download/export — save workspace files to local filesystem

Value

The workspace is the session's scratchpad — plans, notes, artifacts. Currently invisible to users unless they browse ~/.copilot/session-state/ manually.

Priority: 🟡 MEDIUM