ci(.github/workflows): add gofmt check

Adds a `make fmt` target that runs `gofmt -w .` and a matching
`fmt` job in the existing Lint workflow. The job runs `make fmt` and
fails via the existing `scripts/check_unstaged.sh` when any Go file
is not formatted.

Generated with Coder Agents.

Author: zedkipp

.github/workflows/lint.yml

@@ -35,4 +35,17 @@ jobs:
             ${{ env.LINT_CACHE_DIR }}
           key: golangci-lint-${{ runner.os }}-${{ hashFiles('**/*.go') }}
           restore-keys: |
-            golangci-lint-${{ runner.os }}-
+            golangci-lint-${{ runner.os }}-
+
+  fmt:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v5.0.0
+      - uses: actions/setup-go@v5
+        with:
+          go-version: 1.22.8
+      - name: make fmt
+        run: make fmt
+      - name: Check for unstaged files
+        run: ./scripts/check_unstaged.sh

Makefile

@@ -5,4 +5,8 @@ gen:
 
 .PHONY: clean-testdata
 clean-testdata:
-	git clean -xfd testdata
+	git clean -xfd testdata
+
+.PHONY: fmt
+fmt:
+	gofmt -w .