Create CI workflow file

.github/workflows/ci.yml

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+---
+name: CI
+
+# Base file: https://fly.io/phoenix-files/github-actions-for-elixir-ci/#getting-started
+
+"on":
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - "*"
+  workflow_dispatch: {}
+  merge_group:
+
+# Sets the ENV `MIX_ENV` to `test` for running tests
+env:
+  MIX_ENV: test
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{github.workflow}}-${{github.ref}}
+  cancel-in-progress: true
+
+jobs:
+  ci:
+    name: CI OTP-${{matrix.otp-version}} / Elixir ${{matrix.elixir-version}}
+
+    runs-on: ${{matrix.os}}
+
+    strategy:
+      matrix:
+        otp-version: [27]
+        elixir-version: ['1.17.2']
+        os: [ubuntu-24.04]
+
+    steps:
+      # Step: Setup Elixir + Erlang image as the base.
+      - name: Set up Elixir
+      - uses: erlef/setup-beam@b9c58b0450cd832ccdb3c17cc156a47065d2114f # v1.18.1
+        id: setup-beam
+        with:
+          otp-version: ${{matrix.otp-version}}
+          elixir-version: ${{matrix.elixir-version}}
+
+      # Step: Check out the code.
+      - name: Checkout code
+      - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      # Step: Define how to cache deps. Restores existing cache if present.
+      - name: Cache deps
+        id: cache-deps
+        uses: actions/cache@v3
+        env:
+          cache-name: cache-elixir-deps
+        with:
+          path: deps
+          key: ${{runner.os}}-mix-${{env.cache-name}}-${{hashFiles('**/mix.lock')}}
+          restore-keys: |
+            ${{runner.os}}-mix-${{env.cache-name}}-
+
+      # Step: Define how to cache the `_build` directory. After the first run,
+      # this speeds up tests runs a lot. This includes not re-compiling our
+      # project's downloaded deps every run.
+      - name: Cache compiled build
+        id: cache-build
+        uses: actions/cache@v3
+        env:
+          cache-name: cache-compiled-build
+        with:
+          path: _build
+          key: ${{runner.os}}-mix-${{env.cache-name}}-${{hashFiles('**/mix.lock')}}
+          restore-keys: |
+            ${{runner.os}}-mix-${{env.cache-name}}-
+            ${{runner.os}}-mix-
+
+      # Step: Download project dependencies. If unchanged, uses the cached version.
+      - name: Install dependencies
+        run: mix deps.get
+
+      # Step: Compile the project treating any warnings as errors.
+      # Customize this step if a different behavior is desired.
+      - name: Compiles without warnings
+        run: mix compile --warnings-as-errors
+
+      # Step: Check that the checked in code has already been formatted.
+      # This step fails if something was found unformatted.
+      # Customize this step as desired.
+      - name: Check Formatting
+        run: mix format --check-formatted
+
+      # Step: Execute the tests.
+      - name: Run tests
+        run: mix test
+
+      - name: Check if build left artifacts
+        run: |
+          mix deps.unlock --all
+          mix deps.update --all
+          git diff --exit-code