Add minimal CI workflow

Add a GitHub Actions workflow that installs requirements on Python
3.12 and byte-compiles the bot package on every push to main and on
pull requests, catching dependency-resolution and syntax errors early.

.github/workflows/ci.yml

@@ -0,0 +1,25 @@
+name: CI
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+
+      - name: Byte-compile sources
+        run: python -m compileall bot