fix: routing ip rule missing after reboot, busybox compat, skill restructure

Routing fixes (2026-07-14):
- Add missing ip rule 'from 10.11.13.0/24 lookup 100' to hotplug ifup case
- Add UCI network rule persistence so netifd restores it on boot
- Verify ip rule exists in zt-gateway-switch do_force/do_graceful
- Fix BRIDGE_PORTS auto-detect: use /proc/net/dev instead of broken
  awk-over-ip pipeline (busybox awk mishandles exit in compound if)
- Validate bridge port candidate exists as network interface
- Fix setup-routing: use dev br-zt not dev ztX (ZT iface has no IP
  when enslaved to bridge, causing 'Nexthop has invalid gateway')
- Replace ip rule replace (GNU-only) with del+add for busybox

Infrastructure:
- Fix deploy:install stdin starvation: ssh/scp consume pipe data in
  find|while loop; add </dev/null to prevent truncation
- Move luci-dev skill from root/ to skills/ with .agents/skills/ symlink
- Add policy routing and busybox gotcha sections to SKILL.md
- Add diagnostics doc for the routing fix session

.agents/skills/luci-dev

@@ -0,0 +1 @@
+../../skills/luci-dev