Add Session Filesystem Provider (`ISessionFsHandler`)

[PureWeen]

Summary

SDK v0.2.2 added ISessionFsHandler — an interface with 10 filesystem methods that lets you intercept and virtualize file operations for a session. This could enable sandboxed file access for mobile sessions.

SDK API

• ISessionFsHandler (v0.2.2) — 10 filesystem methods
• ServerRpc.SessionFs.SetProviderAsync() — set the filesystem provider

Use Cases

1. Mobile sandboxing — virtualize file operations so mobile sessions don't need real filesystem access
2. Preview changes — intercept writes to show diffs before committing
3. Remote workspace — provide filesystem access to remote repos without cloning locally

Priority: 🟢 LOW