fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/signserver/sign_server_test.go

@@ -329,6 +329,7 @@ func TestServerMutex(t *testing.T) {
 	s := &Server{}
 
 	s.Lock()
+	//nolint:staticcheck // SA2001: testing that Lock/Unlock doesn't panic
 	s.Unlock()
 
 	done := make(chan bool)
@@ -342,6 +343,7 @@ func TestServerMutex(t *testing.T) {
 	time.Sleep(5 * time.Millisecond)
 
 	s.Lock()
+	//nolint:staticcheck // SA2001: testing that Lock/Unlock doesn't panic
 	s.Unlock()
 
 	<-done