fix: confine MCP agents' files to allowed folders; recovery discard deletes only recovery entries (#205) (#578)

Agent tool calls (MCP, and bridged control requests carrying allow_paths) reach files only in the open design's folder (strictly inside home or the temporary folder, not in a hidden folder, and never its hidden files), the app's recovery folder, the server's private scratch folder and each --allow-path; --allow-any-path lifts the limit. Paths are resolved first, so .. and symlinks that lead out (including a save through a link, #845) are refused. On Windows, UNC, device, NT, verbatim non-drive, drive-relative, alternate-stream, reserved-device and 8.3 short-name paths are refused before any file system call, and the resolved path must be on a local drive. doc.recovery_discard and doc.recover {discard} delete only real recovery entries. Docs: mcp.md, control-protocol.md, README.

crates/io/src/design/thumbnail.rs

@@ -60,6 +60,7 @@ pub fn read_design_thumbnail(bytes: &[u8]) -> Option<Vec<u8>> {
 pub fn read_design_thumbnail_file(path: &str) -> Option<Vec<u8>> {
     #[cfg(not(target_arch = "wasm32"))]
     let source = {
+        crate::vfs::check(path).ok()?;
         let metadata = std::fs::metadata(path).ok()?;
         if !metadata.is_file() || metadata.len() > MAX_DESIGN_BYTES as u64 {
             return None;