chore: rebrand backone names, add lint gate + agent docs

- CI: build/validate use backone selftest + binary names, add lint.yml
- make-*.mk: emit/install backone binaries, drop zerotier-one paths
- doc/debian/ext.installfiles: backone manpages, service, init, te
- rule-compiler: npm test runs test.js; attic world planet files
- AGENTS.md repository guidelines, tools/lint.sh changed-lines gate
- gitignore: /backone-selftest, *.gcno, *.gcda

Makefile

@@ -27,6 +27,9 @@ ifeq ($(OSTYPE),NetBSD)
 	include make-netbsd.mk
 endif
 
+lint:
+	@./tools/lint.sh
+
 drone:
 	@echo "rendering .drone.yaml from .drone.jsonnet"
 	drone jsonnet --format --stream