feat: implement Tier 3 utility IRC commands

Implement all 7 utility IRC commands from issue #87:

User commands:
- USERHOST: quick lookup of user@host for up to 5 nicks (RPL 302)
- VERSION: server version string using globals.Version (RPL 351)
- ADMIN: server admin contact info (RPL 256-259)
- INFO: server software info text (RPL 371/374)
- TIME: server local time in RFC format (RPL 391)

Oper commands:
- KILL: forcibly disconnect a user (requires is_oper), broadcasts
  QUIT to all shared channels, cleans up sessions
- WALLOPS: broadcast message to all users with +w usermode
  (requires is_oper)

Supporting changes:
- Add is_wallops column to sessions table in 001_initial.sql
- Add user mode +w tracking via MODE nick +w/-w
- User mode queries now return actual modes (+o, +w)
- MODE -o allows de-opering yourself; MODE +o rejected
- MODE for other users returns ERR_USERSDONTMATCH (502)
- Extract dispatch helpers to reduce dispatchCommand complexity

Tests cover all commands including error cases, oper checks,
user mode set/unset, KILL broadcast, WALLOPS delivery, and
edge cases (self-kill, nonexistent users, missing params).

closes #87

pkg/irc/commands.go

@@ -2,25 +2,32 @@ package irc
 
 // IRC command names (RFC 1459 / RFC 2812).
 const (
-	CmdAway    = "AWAY"
-	CmdInvite  = "INVITE"
-	CmdJoin    = "JOIN"
-	CmdKick    = "KICK"
-	CmdList    = "LIST"
-	CmdLusers  = "LUSERS"
-	CmdMode    = "MODE"
-	CmdMotd    = "MOTD"
-	CmdNames   = "NAMES"
-	CmdNick    = "NICK"
-	CmdNotice  = "NOTICE"
-	CmdOper    = "OPER"
-	CmdPass    = "PASS"
-	CmdPart    = "PART"
-	CmdPing    = "PING"
-	CmdPong    = "PONG"
-	CmdPrivmsg = "PRIVMSG"
-	CmdQuit    = "QUIT"
-	CmdTopic   = "TOPIC"
-	CmdWho     = "WHO"
-	CmdWhois   = "WHOIS"
+	CmdAdmin    = "ADMIN"
+	CmdAway     = "AWAY"
+	CmdInfo     = "INFO"
+	CmdInvite   = "INVITE"
+	CmdJoin     = "JOIN"
+	CmdKick     = "KICK"
+	CmdKill     = "KILL"
+	CmdList     = "LIST"
+	CmdLusers   = "LUSERS"
+	CmdMode     = "MODE"
+	CmdMotd     = "MOTD"
+	CmdNames    = "NAMES"
+	CmdNick     = "NICK"
+	CmdNotice   = "NOTICE"
+	CmdOper     = "OPER"
+	CmdPass     = "PASS"
+	CmdPart     = "PART"
+	CmdPing     = "PING"
+	CmdPong     = "PONG"
+	CmdPrivmsg  = "PRIVMSG"
+	CmdQuit     = "QUIT"
+	CmdTime     = "TIME"
+	CmdTopic    = "TOPIC"
+	CmdUserhost = "USERHOST"
+	CmdVersion  = "VERSION"
+	CmdWallops  = "WALLOPS"
+	CmdWho      = "WHO"
+	CmdWhois    = "WHOIS"
 )