Add ADR for build pipeline consolidation

docs/adr/001-consolidate-build-pipeline.md

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+# ADR 001: Consolidate Build Pipeline
+
+## Status
+
+Proposed
+
+## Context
+
+The theme currently uses two separate build systems:
+
+1. **Grunt** (`Gruntfile.js`) — orchestrates SCSS compilation (`grunt-sass`), CSS post-processing (`autoprefixer`, `postcss-pxtorem`), and JS minification (`terser`)
+2. **Webpack** (`webpack.config.js`) — handles JS bundling and Babel transpilation
+
+This creates maintenance overhead (two configs, two sets of plugins) and increases contributor onboarding friction.
+
+## Decision
+
+Consolidate into a single Webpack-based pipeline that handles:
+
+- SCSS compilation via `sass-loader`
+- CSS post-processing via `postcss-loader` (autoprefixer, pxtorem)
+- JS bundling and transpilation via `babel-loader` (already in use)
+- JS minification via `terser-webpack-plugin`
+
+## Consequences
+
+### Positive
+
+- Single build config (`webpack.config.js`)
+- Remove `Gruntfile.js` and Grunt-specific dependencies
+- Enable tree-shaking for JS bundles
+- Consistent source maps across CSS and JS
+- Fewer dev dependencies to maintain
+
+### Negative
+
+- Migration effort required
+- Docker build commands (`docker-compose.yml`) need updating
+- Must verify output compatibility with Drupal library definitions
+
+### Risks
+
+- CSS output must remain identical to avoid visual regressions
+- The `postcss-pxtorem` plugin behavior must be preserved
+- Drupal.org packaging requirements must still be met (artifacts committed to repo)
+
+## Alternatives Considered
+
+- **Vite**: Faster dev builds but less Drupal ecosystem precedent
+- **Keep Grunt, remove Webpack**: Grunt is effectively unmaintained