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/handlers/api_test.go

@@ -12,6 +12,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync"
 	"testing"
@@ -467,8 +468,11 @@ func findNumeric(
 	msgs []map[string]any,
 	numeric string,
 ) bool {
+	want, _ := strconv.Atoi(numeric)
+
 	for _, msg := range msgs {
-		if msg[commandKey] == numeric {
+		code, ok := msg["code"].(float64)
+		if ok && int(code) == want {
 			return true
 		}
 	}