chore: repo policy compliance sweep — test rerun, npx, frozen lockfile, docs (closes #166)

- script/test runs the suite quietly, then reruns it with --verbose on
  failure and always exits 1 (REPO_POLICIES.md conditional verbose rerun
  pattern). New package.json script test:verbose is the -v form of the
  existing jest --forceExit invocation.
- build.js calls node_modules/.bin/tailwindcss instead of npx, which
  would fetch from the registry unpinned if the binary were absent.
- make install uses --frozen-lockfile, so a stale yarn.lock fails the
  target instead of being silently rewritten.
- README Getting Started uses make setup (which also installs the
  pre-commit hook); the Makefile-only targets (install, hooks, build,
  build-debug, clean, dev) are now documented in Entrypoints.
- .dockerignore records why .git is deliberately not excluded.

script/test

@@ -7,7 +7,13 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 main() {
     cd "$ROOT"
     echo "Running tests..."
-    timeout 30 yarn run test 2>&1
+    timeout 30 yarn run test 2>&1 || {
+        echo "--- Rerunning with --verbose for details ---"
+        timeout 30 yarn run test:verbose 2>&1 || true
+        # Always fail: the first run already proved the tests are broken, so a
+        # flaky pass on the rerun must not turn the build green.
+        exit 1
+    }
 }
 
 main "$@"