ci: add GitHub Actions CI workflow

Runs lint and build on push to develop/main and on PRs.
Uses GH_PAT secret for private @c2s/payload-contracts dependency.

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

.github/workflows/ci.yml

@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  push:
+    branches: [develop, main]
+  pull_request:
+    branches: [develop]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Configure git for private dependencies
+        run: git config --global url."https://x-access-token:${{ secrets.GH_PAT }}@github.com/".insteadOf "github:"
+
+      - uses: pnpm/action-setup@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Lint
+        run: pnpm lint
+
+      - name: Build
+        run: pnpm build