Fix CI workflows to use Yarn 4 (Berry) with corepack

- Add packageManager field to package.json for corepack
- Add .yarnrc.yml with node-modules linker
- Update all workflows to use corepack enable + yarn --immutable
- Remove yarn cache from actions/setup-node (incompatible with Yarn 4)

.github/workflows/ci.yml

@@ -18,10 +18,12 @@ jobs:
         uses: actions/setup-node@v4
         with:
           node-version: '18'
-          cache: 'yarn'
+
+      - name: Enable Corepack
+        run: corepack enable
 
       - name: Install dependencies
-        run: yarn install --frozen-lockfile
+        run: yarn install --immutable
 
       - name: Generate types
         run: yarn codegen