fix(06-03): replace pnpm/action-setup with corepack for Gitea Actions compatibility

pnpm/action-setup@v4 fails in Gitea Actions runners. Use corepack (built into Node.js)
to activate pnpm@9.15.0 matching the packageManager field.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -11,14 +11,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: pnpm/action-setup@v4
-        with:
-          version: 9
-
       - uses: actions/setup-node@v4
         with:
           node-version: 24
-          cache: pnpm
+
+      - name: Enable pnpm via corepack
+        run: corepack enable && corepack prepare pnpm@9.15.0 --activate
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
@@ -36,14 +34,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: pnpm/action-setup@v4
-        with:
-          version: 9
-
       - uses: actions/setup-node@v4
         with:
           node-version: 24
-          cache: pnpm
+
+      - name: Enable pnpm via corepack
+        run: corepack enable && corepack prepare pnpm@9.15.0 --activate
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile