feat: add IRC wire protocol listener with shared service layer

Adds a backward-compatible IRC wire protocol listener (RFC 1459/2812)
with a shared service layer used by both IRC and HTTP transports.

- TCP listener on configurable port (default :6667)
- Full IRC protocol: NICK, USER, JOIN, PART, PRIVMSG, MODE, TOPIC, etc.
- Shared service layer (internal/service/) for consistent code paths
- Tier 2 join restrictions (ban, invite-only, key, limit) in service layer
- Ban check on PRIVMSG in service layer
- SetChannelFlag handles +i and +s modes
- Command dispatch via map[string]cmdHandler pattern
- EXPOSE 6667 in Dockerfile
- Service layer unit tests

closes #89

pkg/irc/commands.go

@@ -21,6 +21,7 @@ const (
 	CmdPrivmsg = "PRIVMSG"
 	CmdQuit    = "QUIT"
 	CmdTopic   = "TOPIC"
+	CmdUser    = "USER"
 	CmdWho     = "WHO"
 	CmdWhois   = "WHOIS"
 )