build(lint): migrate golangci-lint config to v2 schema

Installed golangci-lint v2 rejected the v1 config (unsupported version), silently disabling the lint gate. Migrate to v2 (version, formatters section, linters.exclusions) preserving the same linter selection so the gate runs again.

.golangci.yml

@@ -1,4 +1,14 @@
-linters-settings:
+version: "2"
+
+linters:
+  enable:
+    - errcheck
+    - gosec
+    - govet
+    - staticcheck
+    - unused
+    - ineffassign
+  settings:
     govet:
       enable:
         - fieldalignment
@@ -8,28 +18,20 @@ linters-settings:
         - (io.Closer).Close
         - (*encoding/json.Encoder).Encode
         - (io.Writer).Write
-
+  exclusions:
-linters:
+    paths:
-  enable:
-    - errcheck
-    - gosec
-    - govet
-    - gofmt
-    - staticcheck
-    - unused
-    - ineffassign
-    - typecheck
-
-issues:
-  exclude-dirs:
       - vendor
     # Exclude some linters from running on test files
-  exclude-rules:
+    rules:
       - path: _test\.go
         linters:
           - errcheck
           - gosec
 
+formatters:
+  enable:
+    - gofmt
+
 run:
   timeout: 5m
   tests: true