docs: add doc.go files and godoc comments to all packages

Add package-level documentation (doc.go) to all 22 first-party
packages and godoc comments to ~150 previously undocumented
exported symbols across common/, network/, and server/.

server/channelserver/handlers_guild_mission.go

@@ -5,6 +5,7 @@ import (
 	"erupe-ce/network/mhfpacket"
 )
 
+// GuildMission represents a guild mission entry.
 type GuildMission struct {
 	ID          uint32
 	Unk         uint32