docs: initial plan for ZeroTier exit gateway switcher

Design: .omp/plans/zerotier-gateway-switching.md — LuCI app + rpcd/ucode backend + zt-gateway-switch shell script for force / graceful-drain gateway switching on OpenWRT.

.gitignore

@@ -0,0 +1,28 @@
+# OpenWRT / LuCI build artifacts
+bin/
+tmp/
+.build/
+.config
+.config.old
+feeds.conf.default
+feeds.conf
+*.ipk
+*.ipk.opk
+
+# SDK / build cache
+dl/
+staging_dir*/
+build_dir*/
+
+# Translations (compiled)
+*.po~
+*.pot
+
+# Common OS / editor files
+.DS_Store
+Thumbs.db
+*.swp
+*.swo
+*~
+.vscode/
+.idea/