Make CI and agents run rustfmt

.github/workflows/ci.yml

@@ -120,6 +120,47 @@ jobs:
           . "$HOME/.cargo/env"
           cargo clippy --all-targets -- -D warnings
 
+  fmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    container:
+      image: ubuntu:25.10
+    steps:
+      - name: Install git
+        run: |
+          apt-get update
+          apt-get install -y git curl
+
+      - uses: actions/checkout@v4
+
+      - name: Install build dependencies
+        run: ./scripts/shepherd deps install build
+
+      - name: Add rustfmt component
+        run: |
+          . "$HOME/.cargo/env"
+          rustup component add rustfmt
+
+      - name: Add Rust to PATH
+        run: echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
+      - name: Cache cargo registry and build
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-
+
+      - name: Check formatting
+        run: |
+          . "$HOME/.cargo/env"
+          cargo fmt --all
+          git diff --exit-code
+
   shellcheck:
     name: ShellCheck
     runs-on: ubuntu-latest

AGENTS.md

@@ -2,7 +2,9 @@ Agents: please use the existing documentation for setup.
 
 <CONTRIBUTING.md> describes environment setup and build, test, and lint, including helper scripts and exact commands.
 
-Please ensure that your changes build and pass tests and lint. If you changed the example configuration at <config.example.toml>, make sure that it passes config validation.
+Please ensure that your changes build and pass tests and lint, and run `cargo fmt --all` to match your changes to the rest of the code.
+
+If you changed the example configuration at <config.example.toml>, make sure that it passes config validation.
 
 Each of the Rust crates in <crates> contains a README.md that describes each at a high level.