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/commands.go

@@ -0,0 +1,21 @@
+package irc
+
+// IRC command names (RFC 1459 / RFC 2812).
+const (
+	CmdJoin    = "JOIN"
+	CmdList    = "LIST"
+	CmdLusers  = "LUSERS"
+	CmdMode    = "MODE"
+	CmdMotd    = "MOTD"
+	CmdNames   = "NAMES"
+	CmdNick    = "NICK"
+	CmdNotice  = "NOTICE"
+	CmdPart    = "PART"
+	CmdPing    = "PING"
+	CmdPong    = "PONG"
+	CmdPrivmsg = "PRIVMSG"
+	CmdQuit    = "QUIT"
+	CmdTopic   = "TOPIC"
+	CmdWho     = "WHO"
+	CmdWhois   = "WHOIS"
+)