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

cmd/neoirc-cli/main.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	api "git.eeqj.de/sneak/neoirc/cmd/neoirc-cli/api"
+	"git.eeqj.de/sneak/neoirc/internal/irc"
 )
 
 const (
@@ -16,17 +17,6 @@ const (
 	pollTimeout = 15
 	pollRetry   = 2 * time.Second
 	timeFormat  = "15:04"
-
-	cmdJoin    = "JOIN"
-	cmdMotd    = "MOTD"
-	cmdNick    = "NICK"
-	cmdNotice  = "NOTICE"
-	cmdPart    = "PART"
-	cmdPrivmsg = "PRIVMSG"
-	cmdQuit    = "QUIT"
-	cmdTopic   = "TOPIC"
-	cmdWho     = "WHO"
-	cmdWhois   = "WHOIS"
 )
 
 // App holds the application state.
@@ -97,7 +87,7 @@ func (a *App) handleInput(text string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: cmdPrivmsg,
+		Command: irc.CmdPrivmsg,
 		To:      target,
 		Body:    []string{text},
 	})
@@ -252,7 +242,7 @@ func (a *App) cmdNick(nick string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: cmdNick,
+		Command: irc.CmdNick,
 		Body:    []string{nick},
 	})
 	if err != nil {
@@ -387,7 +377,7 @@ func (a *App) cmdMsg(args string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: cmdPrivmsg,
+		Command: irc.CmdPrivmsg,
 		To:      target,
 		Body:    []string{text},
 	})
@@ -445,7 +435,7 @@ func (a *App) cmdTopic(args string) {
 
 	if args == "" {
 		err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-			Command: cmdTopic,
+			Command: irc.CmdTopic,
 			To:      target,
 		})
 		if err != nil {
@@ -458,7 +448,7 @@ func (a *App) cmdTopic(args string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: cmdTopic,
+		Command: irc.CmdTopic,
 		To:      target,
 		Body:    []string{args},
 	})
@@ -546,7 +536,7 @@ func (a *App) cmdMotd() {
 	}
 
 	err := a.client.SendMessage(
-		&api.Message{Command: cmdMotd}, //nolint:exhaustruct
+		&api.Message{Command: irc.CmdMotd}, //nolint:exhaustruct
 	)
 	if err != nil {
 		a.ui.AddStatus(fmt.Sprintf(
@@ -583,7 +573,7 @@ func (a *App) cmdWho(args string) {
 
 	err := a.client.SendMessage(
 		&api.Message{ //nolint:exhaustruct
-			Command: cmdWho, To: channel,
+			Command: irc.CmdWho, To: channel,
 		},
 	)
 	if err != nil {
@@ -614,7 +604,7 @@ func (a *App) cmdWhois(args string) {
 
 	err := a.client.SendMessage(
 		&api.Message{ //nolint:exhaustruct
-			Command: cmdWhois, To: args,
+			Command: irc.CmdWhois, To: args,
 		},
 	)
 	if err != nil {
@@ -664,7 +654,7 @@ func (a *App) cmdQuit() {
 
 	if a.connected && a.client != nil {
 		_ = a.client.SendMessage(
-			&api.Message{Command: cmdQuit}, //nolint:exhaustruct
+			&api.Message{Command: irc.CmdQuit}, //nolint:exhaustruct
 		)
 	}
 
@@ -749,19 +739,19 @@ func (a *App) handleServerMessage(msg *api.Message) {
 	a.mu.Unlock()
 
 	switch msg.Command {
-	case cmdPrivmsg:
+	case irc.CmdPrivmsg:
 		a.handlePrivmsgEvent(msg, timestamp, myNick)
-	case cmdJoin:
+	case irc.CmdJoin:
 		a.handleJoinEvent(msg, timestamp)
-	case cmdPart:
+	case irc.CmdPart:
 		a.handlePartEvent(msg, timestamp)
-	case cmdQuit:
+	case irc.CmdQuit:
 		a.handleQuitEvent(msg, timestamp)
-	case cmdNick:
+	case irc.CmdNick:
 		a.handleNickEvent(msg, timestamp, myNick)
-	case cmdNotice:
+	case irc.CmdNotice:
 		a.handleNoticeEvent(msg, timestamp)
-	case cmdTopic:
+	case irc.CmdTopic:
 		a.handleTopicEvent(msg, timestamp)
 	default:
 		a.handleDefaultEvent(msg, timestamp)