Publish to the GitLab package registry on version tags

The README's install line failed: the package was never published, and
nothing told npm where the @extant2000 scope lives. A tag like v1.2.3 now
publishes to this project's registry with CI_JOB_TOKEN, and the README
sets the scope's registry before installing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitlab-ci.yml

@@ -1,6 +1,21 @@
-stages: [test]
+stages: [test, publish]
 
 test:
   stage: test
   image: node:22-alpine
   script: [npm ci, npm run build, npm test]
+
+# Publishes to this project's package registry on a version tag (v1.2.3).
+# CI_JOB_TOKEN has write access to the project's own registry, so no stored
+# secret is needed.
+publish:
+  stage: publish
+  image: node:22-alpine
+  rules:
+    - if: $CI_COMMIT_TAG =~ /^v\d+\.\d+\.\d+$/
+  script:
+    - npm ci
+    - npm run build
+    - echo "@extant2000:registry=${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/packages/npm/" > .npmrc
+    - echo "${CI_API_V4_URL#https:}/projects/${CI_PROJECT_ID}/packages/npm/:_authToken=${CI_JOB_TOKEN}" >> .npmrc
+    - npm publish