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

.github/workflows/lint.yml

@@ -0,0 +1,25 @@
+name: Lint
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  clang-format:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      # Pin: format results differ across clang-format versions.
+      - run: pip install --user clang-format==23.1.1 && echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+      - name: Check changed lines
+        run: |
+          if [ "${{ github.event_name }}" = "pull_request" ]; then
+            BASE="${{ github.event.pull_request.base.sha }}"
+          elif [ "${{ github.event.before }}" != "" ] && [ "${{ github.event.before }}" != "0000000000000000000000000000000000000000" ]; then
+            BASE="${{ github.event.before }}"
+          else
+            BASE="$(git merge-base HEAD "origin/${{ github.event.repository.default_branch }}" || echo HEAD~1)"
+          fi
+          ./tools/lint.sh "$BASE"