feat: route staged scene skills through runtime

Add registry-driven scene routing and multi-root skill loading so fault-details and 95598 scene skills can be triggered from natural language while still running through the browser-backed runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/task_runner_test.rs

@@ -1,5 +1,7 @@
 mod common;
 
+use std::fs;
+use std::path::PathBuf;
 use std::sync::{Arc, Mutex, OnceLock};
 use std::time::Duration;