fix: check all Close() return values for errcheck lint

Add explicit error discards (_ =) for Close() calls on network
connections, SQL rows, and file handles across 28 files. Also add
.golangci.yml with standard linter defaults to match CI configuration.

server/channelserver/handlers_guild_tresure.go

@@ -50,7 +50,7 @@ func handleMsgMhfEnumerateGuildTresure(s *Session, p mhfpacket.MHFPacket) {
 			WHERE gh.guild_id=$2 AND gh.level=2 AND gh.acquired=TRUE
 		`, s.charID, guild.ID)
 		if err != nil {
-			rows.Close()
+			_ = rows.Close()
 			doAckBufSucceed(s, pkt.AckHandle, make([]byte, 4))
 			return
 		} else {