chore: add nightly CI workflow

Adds a GitHub Actions workflow that runs the basic `moon check` and
`moon test` pipeline against the MoonBit nightly channel on every push
and pull request.

Generated with Claude Code (https://claude.com/claude-code).

Author: bobzhang

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+
+jobs:
+  check-and-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Set up MoonBit (nightly)
+        run: |
+          curl -fsSL https://cli.moonbitlang.com/install/unix.sh | bash -s nightly
+          echo "$HOME/.moon/bin" >> $GITHUB_PATH
+
+      - name: Show MoonBit version
+        run: moon version --all
+
+      - name: Update dependencies
+        run: moon update
+
+      - name: Check
+        run: moon check
+
+      - name: Test
+        run: moon test