Update golangci-lint and tighten lint config (#190)

Bump `golangci-lint` to v2.12.2 and pin `govulncheck` to v1.3.0. Align `.golangci.yml` with the gitea repo's config: enable `revive` `var-naming` (with `skip-package-name-checks`) and drop the test-file exclusion for `errcheck`/`staticcheck`/`unparam`. Fix the now-surfaced `errcheck` violations in test handlers by discarding return values to match the existing codebase pattern.

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/190
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

pkg/gitea/redirect_test.go

@@ -99,7 +99,7 @@ func TestDoJSON_GETRedirectFollowed(t *testing.T) {
 	mux.HandleFunc("GET /api/v1/repos/owner/new-name/pulls/1", func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/json")
 		w.WriteHeader(http.StatusOK)
-		json.NewEncoder(w).Encode(map[string]any{"id": 1, "title": "found"})
+		_ = json.NewEncoder(w).Encode(map[string]any{"id": 1, "title": "found"})
 	})
 	srv := httptest.NewServer(mux)
 	defer srv.Close()