fix(openwrt): use full opkg path and pre-check availability

`channel.exec()` doesn't source the shell profile, so PATH may not
include /usr/bin on some routers. Using /usr/bin/opkg explicitly
avoids exit-127 surprises. Added opkg_check() to give a clear error
("firmware may not support package management") before attempting
opkg_update, rather than a confusing "command not found" exit code.
Also split the BusyBox-hostile `grep -v 'all\|noarch'` into two
separate greps for the arch-detection fallback.

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

core/openwrt/src/tollgate/mod.rs

@@ -19,6 +19,7 @@ use crate::Router;
 pub async fn provision(router: &Router, config: &TollGateConfig) -> Result<()> {
     info!("[{}] Starting TollGate provisioning", router.host);
 
+    router.opkg_check()?;
     router.opkg_update()?;
     install_tollgate(router)?;
     config::apply(router, config)?;