ci: migrate golangci-lint to v2 (#12)

golangci-lint v1.55 (action@v2) can't run against the module's Go
version, and the v1 .golangci.toml is rejected by current golangci-lint
("unsupported version of the configuration").

- Migrate the config to the v2 schema (.golangci.yml). Same linter set;
  gofmt/goimports move to `formatters`, vet->govet (+shadow).
- Bump golangci-lint-action v2 -> v9, pin golangci-lint v2.11.4, add a
  setup-go step on stable, and update actions/checkout v2 -> v7.

Code passes the migrated config with 0 issues.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: reuvenharrison

.github/workflows/lint.yaml

@@ -13,9 +13,14 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v7
+
+      - name: Set up Go
+        uses: actions/setup-go@v6
+        with:
+          go-version: stable
 
       - name: Lint
-        uses: golangci/golangci-lint-action@v2
+        uses: golangci/golangci-lint-action@v9
         with:
-          version: v1.55
+          version: v2.11.4

.golangci.toml

@@ -0,0 +1,26 @@
+version: "2"
+
+run:
+  timeout: 120s
+
+linters:
+  enable:
+    - errcheck
+    - goconst
+    - gocyclo
+    - govet
+    - ineffassign
+    - nakedret
+    - revive
+    - unconvert
+    - unparam
+    - unused
+  settings:
+    govet:
+      enable:
+        - shadow
+
+formatters:
+  enable:
+    - gofmt
+    - goimports