Remove yarn, use npm exclusively

Replace all yarn references with npm across README, CLAUDE.md,
CHANGELOG, .gitignore, and CI workflow. Remove yarn.lock.

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

CLAUDE.md

@@ -44,7 +44,7 @@ Next milestone is **v0.2.0 - Design Editor**. See `PLAN.md` for details.
 - **Template Engine:** Twig 3.0 (via Composer)
 - **Frontend:** Bootstrap 5 Javascript & Vanilla JavaScript
 - **Styling:** Bootstrap 5 & Custom CSS (if necessary)
-- **Dependency Management:** Composer (PHP), npm/Yarn (JS/CSS)
+- **Dependency Management:** Composer (PHP), npm (JS/CSS)
 - **Internationalization:** WordPress i18n (.pot/.po/.mo files)
 - **Canonical Plugin Name:** `wp-bootstrap`
 
@@ -178,7 +178,7 @@ Build steps (in order):
 3. `scss` — Compile SCSS (`src/scss/`) to CSS (`assets/css/`)
 4. `postcss` — Autoprefixer + cssnano minification → `assets/css/style.min.css`
 
-**CI/CD note:** The Gitea workflow uses `npm install` and `npm run build` (not `yarn`). The CI runner does not have `yarn` available.
+**CI/CD note:** The Gitea workflow uses `npm install` and `npm run build`.
 
 ### Architecture Notes
 
@@ -205,7 +205,7 @@ Build steps (in order):
 
 **Key learnings:**
 
-- `actions/setup-node@v4` does not install `yarn` — use `npm install` / `npm run build` in CI workflows
+- CI uses `npm install` / `npm run build` — `yarn` is not available in the CI runner
 - Bootstrap 5 alert colors are hardcoded (not CSS variables), so explicit dark mode overrides are needed in SCSS
 - Anti-flash for dark mode requires a synchronous inline script via `wp_add_inline_script('handle', '...', 'before')` — the deferred JS alone causes a flash
 - Variable fonts use `fontWeight: "100 900"` range syntax in `theme.json` `fontFace` declarations