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/sketch/src/fonts.rs

@@ -259,6 +259,13 @@ fn normalize(s: &str) -> String {
     s.chars().filter(|c| c.is_alphanumeric()).flat_map(char::to_lowercase).collect()
 }
 
+/// The file a font name refers to (after `~` and `%VAR%` expansion), or `None` for the built-in
+/// font or an installed family, so callers can vet the path before [`font_data`] reads it.
+pub fn font_file_path(name: &str) -> Option<PathBuf> {
+    let name = name.trim();
+    (is_path(name) && name.len() <= MAX_FONT_NAME).then(|| expand(name))
+}
+
 fn is_path(name: &str) -> bool {
     name.contains('/') || name.contains('\\') || has_font_extension(name)
 }