Add plugin zip build task and CI release artifact

- bin/build-zip.sh + `composer build`: stage runtime files only, generate a
  production (no-dev) optimized autoloader, and emit
  dist/<slug>-<version>.zip with a single top-level plugin folder, ready to
  upload via wp-admin. Tests, tooling configs, docs, and dev dependencies
  are excluded; version is read from the plugin header.
- CI `build` job: on push to main (post-merge), after lint/static-analysis/
  test/no-debug pass, runs the build and uploads the zip via
  actions/upload-artifact.
- Ignore build/ and dist/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

composer.json

@@ -31,7 +31,8 @@
         "test:coverage": "phpunit --configuration phpunit.xml --coverage-html coverage/",
         "lint": "phpstan analyse src/ --level=6 --configuration phpstan.neon",
         "cs": "phpcs --standard=phpcs.xml.dist",
-        "cs:fix": "phpcbf --standard=phpcs.xml.dist"
+        "cs:fix": "phpcbf --standard=phpcs.xml.dist",
+        "build": "bash bin/build-zip.sh"
     },
     "config": {
         "allow-plugins": {