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

Dockerfile

@@ -53,7 +53,7 @@ RUN apk add --no-cache ca-certificates \
 COPY --from=builder /neoircd /usr/local/bin/neoircd
 
 USER neoirc
-EXPOSE 8080
+EXPOSE 8080 6667
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
     CMD wget -qO- http://localhost:8080/.well-known/healthcheck.json || exit 1
 ENTRYPOINT ["neoircd"]