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_adventure.go

@@ -9,6 +9,7 @@ import (
 	"go.uber.org/zap"
 )
 
+// GuildAdventure represents a guild adventure expedition.
 type GuildAdventure struct {
 	ID          uint32 `db:"id"`
 	Destination uint32 `db:"destination"`