ci: add testing and snap release workflows

Testing runs the pre-commit trio on pushes to main and PRs.
Releasing builds the snap on plain vX tags and publishes it to the
stable store channel; rc tags fail the version gate and leave the
publish job skipped.

.github/workflows/ci.yml

@@ -0,0 +1,39 @@
+name: ci
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  test:
+    name: fmt, clippy, test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy, rustfmt
+
+      - uses: Swatinem/rust-cache@v2
+
+      # Same order as the pre-commit list in AGENTS.md.
+      - name: cargo fmt
+        run: cargo fmt --all -- --check
+
+      - name: cargo clippy
+        run: cargo clippy --all-targets -- -D warnings
+
+      - name: cargo test
+        run: cargo test