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/engine/src/tests.rs

@@ -198,6 +198,7 @@ fn join_meeting_nothing_keeps_the_first_body() {
 mod boolean_diagnostics;
 mod component_frames;
 mod mirror_planes;
+mod path_scope;
 mod primitive_points;
 mod robustness;
 mod robustness_goldens;