build(release): split core builds from release-only artifacts

- add a dedicated build-release-artifacts target for Android bundle packaging
- switch CI and release workflows to Corepack-managed pnpm with cache support
- pin the frontend pnpm version and make dependency installs deterministic
- inject version metadata into launcher binaries in GoReleaser
- update build documentation to reflect the new workflow

.github/workflows/release.yml

@@ -69,9 +69,11 @@ jobs:
         uses: actions/setup-node@v6
         with:
           node-version: 22
+          cache: pnpm
+          cache-dependency-path: web/frontend/pnpm-lock.yaml
 
       - name: Setup pnpm
-        run: corepack enable && corepack prepare pnpm@latest --activate
+        run: corepack enable && corepack install
 
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v4
@@ -110,13 +112,13 @@ jobs:
           MACOS_NOTARY_KEY_ID: ${{ secrets.MACOS_NOTARY_KEY_ID }}
           MACOS_NOTARY_KEY: ${{ secrets.MACOS_NOTARY_KEY }}
 
-      - name: Build and upload Android arm64
+      - name: Build and upload release-only artifacts
         shell: bash
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           sudo apt-get install -y zip
-          make build-android-bundle
+          make build-release-artifacts
           gh release upload "${{ inputs.tag }}" \
             build/picoclaw-android-universal.zip \
             --clobber