feat: add irc numerics package, deduplicate constants, fix dead code

- Create internal/irc/ package with all IRC numeric reply codes (RFC 1459/2812)
  and command string constants as the single source of truth
- Replace all 69+ bare numeric string literals in api.go with named constants
  (e.g. irc.RplWelcome, irc.ErrNoSuchChannel)
- Add 'code' (int) and named 'command' (e.g. RPL_YOURHOST) fields to IRC
  message JSON replies via irc.Name() lookup in scanMessages
- Deduplicate command constants: remove local definitions from api.go,
  cmd/neoirc-cli/main.go, and cmd/neoirc-cli/api/client.go; all now import
  from internal/irc
- Fix dead code: remove handleListCmd/handleWhoCmd/handleWhoisCmd/
  sendWhoisNumerics that were unreachable due to dispatchCommand routing
  LIST/WHO/WHOIS to dispatchInfoCommand before dispatchQueryCommand.
  Route these commands to dispatchQueryCommand which has the improved
  implementations (e.g. ListAllChannelsWithCounts single-query vs N+1)
- Update enqueueNumeric and respondIRCError signatures from string to int
- Update test helper findNumeric to check the new 'code' JSON field

Closes #52

internal/irc/numerics.go

@@ -0,0 +1,150 @@
+// Package irc provides constants and utilities for the
+// IRC protocol, including numeric reply codes from
+// RFC 1459 and RFC 2812, and standard command names.
+package irc
+
+// Connection registration replies (001-005).
+const (
+	RplWelcome  = 1
+	RplYourHost = 2
+	RplCreated  = 3
+	RplMyInfo   = 4
+	RplIsupport = 5
+)
+
+// Command responses (200-399).
+const (
+	RplUmodeIs       = 221
+	RplLuserClient   = 251
+	RplLuserOp       = 252
+	RplLuserUnknown  = 253
+	RplLuserChannels = 254
+	RplLuserMe       = 255
+	RplAway          = 301
+	RplUserHost      = 302
+	RplIson          = 303
+	RplUnaway        = 305
+	RplNowAway       = 306
+	RplWhoisUser     = 311
+	RplWhoisServer   = 312
+	RplWhoisOperator = 313
+	RplEndOfWho      = 315
+	RplWhoisIdle     = 317
+	RplEndOfWhois    = 318
+	RplWhoisChannels = 319
+	RplList          = 322
+	RplListEnd       = 323
+	RplChannelModeIs = 324
+	RplCreationTime  = 329
+	RplNoTopic       = 331
+	RplTopic         = 332
+	RplTopicWhoTime  = 333
+	RplInviting      = 341
+	RplWhoReply      = 352
+	RplNamReply      = 353
+	RplEndOfNames    = 366
+	RplBanList       = 367
+	RplEndOfBanList  = 368
+	RplMotd          = 372
+	RplMotdStart     = 375
+	RplEndOfMotd     = 376
+)
+
+// Error replies (400-599).
+const (
+	ErrNoSuchNick        = 401
+	ErrNoSuchServer      = 402
+	ErrNoSuchChannel     = 403
+	ErrCannotSendToChan  = 404
+	ErrTooManyChannels   = 405
+	ErrNoRecipient       = 411
+	ErrNoTextToSend      = 412
+	ErrUnknownCommand    = 421
+	ErrNoNicknameGiven   = 431
+	ErrErroneusNickname  = 432
+	ErrNicknameInUse     = 433
+	ErrUserNotInChannel  = 441
+	ErrNotOnChannel      = 442
+	ErrNotRegistered     = 451
+	ErrNeedMoreParams    = 461
+	ErrAlreadyRegistered = 462
+	ErrChannelIsFull     = 471
+	ErrInviteOnlyChan    = 473
+	ErrBannedFromChan    = 474
+	ErrBadChannelKey     = 475
+	ErrChanOpPrivsNeeded = 482
+)
+
+// names maps numeric codes to their standard IRC names.
+//
+//nolint:gochecknoglobals
+var names = map[int]string{
+	RplWelcome:       "RPL_WELCOME",
+	RplYourHost:      "RPL_YOURHOST",
+	RplCreated:       "RPL_CREATED",
+	RplMyInfo:        "RPL_MYINFO",
+	RplIsupport:      "RPL_ISUPPORT",
+	RplUmodeIs:       "RPL_UMODEIS",
+	RplLuserClient:   "RPL_LUSERCLIENT",
+	RplLuserOp:       "RPL_LUSEROP",
+	RplLuserUnknown:  "RPL_LUSERUNKNOWN",
+	RplLuserChannels: "RPL_LUSERCHANNELS",
+	RplLuserMe:       "RPL_LUSERME",
+	RplAway:          "RPL_AWAY",
+	RplUserHost:      "RPL_USERHOST",
+	RplIson:          "RPL_ISON",
+	RplUnaway:        "RPL_UNAWAY",
+	RplNowAway:       "RPL_NOWAWAY",
+	RplWhoisUser:     "RPL_WHOISUSER",
+	RplWhoisServer:   "RPL_WHOISSERVER",
+	RplWhoisOperator: "RPL_WHOISOPERATOR",
+	RplEndOfWho:      "RPL_ENDOFWHO",
+	RplWhoisIdle:     "RPL_WHOISIDLE",
+	RplEndOfWhois:    "RPL_ENDOFWHOIS",
+	RplWhoisChannels: "RPL_WHOISCHANNELS",
+	RplList:          "RPL_LIST",
+	RplListEnd:       "RPL_LISTEND", //nolint:misspell
+	RplChannelModeIs: "RPL_CHANNELMODEIS",
+	RplCreationTime:  "RPL_CREATIONTIME",
+	RplNoTopic:       "RPL_NOTOPIC",
+	RplTopic:         "RPL_TOPIC",
+	RplTopicWhoTime:  "RPL_TOPICWHOTIME",
+	RplInviting:      "RPL_INVITING",
+	RplWhoReply:      "RPL_WHOREPLY",
+	RplNamReply:      "RPL_NAMREPLY",
+	RplEndOfNames:    "RPL_ENDOFNAMES",
+	RplBanList:       "RPL_BANLIST",
+	RplEndOfBanList:  "RPL_ENDOFBANLIST",
+	RplMotd:          "RPL_MOTD",
+	RplMotdStart:     "RPL_MOTDSTART",
+	RplEndOfMotd:     "RPL_ENDOFMOTD",
+
+	ErrNoSuchNick:        "ERR_NOSUCHNICK",
+	ErrNoSuchServer:      "ERR_NOSUCHSERVER",
+	ErrNoSuchChannel:     "ERR_NOSUCHCHANNEL",
+	ErrCannotSendToChan:  "ERR_CANNOTSENDTOCHAN",
+	ErrTooManyChannels:   "ERR_TOOMANYCHANNELS",
+	ErrNoRecipient:       "ERR_NORECIPIENT",
+	ErrNoTextToSend:      "ERR_NOTEXTTOSEND",
+	ErrUnknownCommand:    "ERR_UNKNOWNCOMMAND",
+	ErrNoNicknameGiven:   "ERR_NONICKNAMEGIVEN",
+	ErrErroneusNickname:  "ERR_ERRONEUSNICKNAME",
+	ErrNicknameInUse:     "ERR_NICKNAMEINUSE",
+	ErrUserNotInChannel:  "ERR_USERNOTINCHANNEL",
+	ErrNotOnChannel:      "ERR_NOTONCHANNEL",
+	ErrNotRegistered:     "ERR_NOTREGISTERED",
+	ErrNeedMoreParams:    "ERR_NEEDMOREPARAMS",
+	ErrAlreadyRegistered: "ERR_ALREADYREGISTERED",
+	ErrChannelIsFull:     "ERR_CHANNELISFULL",
+	ErrInviteOnlyChan:    "ERR_INVITEONLYCHAN",
+	ErrBannedFromChan:    "ERR_BANNEDFROMCHAN",
+	ErrBadChannelKey:     "ERR_BADCHANNELKEY",
+	ErrChanOpPrivsNeeded: "ERR_CHANOPRIVSNEEDED",
+}
+
+// Name returns the standard IRC name for a numeric code
+// (e.g., Name(2) returns "RPL_YOURHOST"). Returns an
+// empty string if the code is unknown.
+func Name(code int) string {
+	return names[code]
+}