fix: improve yarn cache strategy to prevent immutable lockfile errors

- Cache .yarn/cache instead of node_modules to avoid inconsistencies
- Include both yarn.lock and package.json in cache key hash
- Update restore-keys to be more specific with package.json hash
- This prevents CI from using stale caches after dependency updates

https://claude.ai/code/session_015R3U3kiprQiNTTNNt31Sg6

.github/workflows/build.yml

@@ -38,11 +38,11 @@ jobs:
         id: yarn-cache
         with:
           path: |
-            node_modules
+            .yarn/cache
             .yarn/install-state.gz
-          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('yarn.lock', 'package.json') }}
           restore-keys: |
-            ${{ runner.os }}-yarn-
+            ${{ runner.os }}-yarn-${{ hashFiles('package.json') }}-
 
       - name: Install dependencies
         env: