ci: migrate binaries to GitHub Release + Actions CI

- Add .github/workflows/build-binaries.yml (Go cross-compile → preset release)
- installer.ts: download from GitHub Release, fallback to local bin/
- package.json: exclude binaries from npm (16.3MB → 161KB)
- .gitignore: stop tracking bin/codeagent-wrapper-*
- git rm --cached 6 pre-compiled binaries

.github/workflows/build-binaries.yml

@@ -0,0 +1,74 @@
+name: Build codeagent-wrapper binaries
+
+on:
+  push:
+    paths:
+      - 'codeagent-wrapper/**'
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.21'
+
+      - name: Cross-compile binaries
+        working-directory: codeagent-wrapper
+        run: |
+          set -e
+          mkdir -p ../artifacts
+
+          declare -a targets=(
+            "darwin:amd64:"
+            "darwin:arm64:"
+            "linux:amd64:"
+            "linux:arm64:"
+            "windows:amd64:.exe"
+            "windows:arm64:.exe"
+          )
+
+          for target in "${targets[@]}"; do
+            IFS=':' read -r os arch ext <<< "$target"
+            output="codeagent-wrapper-${os}-${arch}${ext}"
+            echo "::group::Building ${output}"
+            GOOS=$os GOARCH=$arch go build -ldflags="-s -w" -o "../artifacts/${output}" .
+            echo "::endgroup::"
+          done
+
+          echo "### Build artifacts" >> $GITHUB_STEP_SUMMARY
+          ls -lh ../artifacts/ | awk 'NR>1{printf "- `%s` (%s)\n", $NF, $5}' >> $GITHUB_STEP_SUMMARY
+
+      - name: Delete existing preset release
+        run: gh release delete preset --yes --cleanup-tag 2>/dev/null || true
+        env:
+          GH_TOKEN: ${{ github.token }}
+
+      - name: Create preset release
+        uses: softprops/action-gh-release@v2
+        with:
+          tag_name: preset
+          name: "Pre-built Binaries (codeagent-wrapper)"
+          body: |
+            Pre-compiled `codeagent-wrapper` binaries for all supported platforms.
+            Automatically downloaded during `npx ccg-workflow` installation.
+
+            | File | Platform | Architecture |
+            |------|----------|-------------|
+            | `codeagent-wrapper-darwin-amd64` | macOS | Intel x64 |
+            | `codeagent-wrapper-darwin-arm64` | macOS | Apple Silicon |
+            | `codeagent-wrapper-linux-amd64` | Linux | x64 |
+            | `codeagent-wrapper-linux-arm64` | Linux | ARM64 |
+            | `codeagent-wrapper-windows-amd64.exe` | Windows | x64 |
+            | `codeagent-wrapper-windows-arm64.exe` | Windows | ARM64 |
+
+            > This release is automatically updated when `codeagent-wrapper/` source changes.
+          files: artifacts/*
+          prerelease: true
+          make_latest: false