feat: simplify installation to single @stable tag

- Remove automatic versioning and dual publishing strategy
- Delete release.yaml and promote-to-stable.yaml workflows
- Add manual-release.yaml for controlled releases
- Remove semantic-release dependencies and config
- Update all documentation to use npx bmad-method install
- Configure NPM to publish to @stable tag by default
- Users can now use simple npx bmad-method install command

README.md

@@ -75,6 +75,8 @@ This makes it easy to benefit from the latest improvements, bug fixes, and new a
 
 ```bash
 npx bmad-method install
+# OR explicitly use stable tag:
+npx bmad-method@stable install
 # OR if you already have BMad installed:
 git pull
 npm run install:bmad