feat(channels): add IRC channel integration

Add IRC as a new channel for picoclaw, supporting server connections,
channel joins, DMs, mention-based group triggers, and IRCv3 typing
indicators. Uses ergochat/irc-go for connection management with SASL,
NickServ, and automatic reconnection support.

Closes #1137

pkg/channels/irc/handler.go

@@ -0,0 +1,140 @@
+package irc
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/ergochat/irc-go/ircevent"
+	"github.com/ergochat/irc-go/ircmsg"
+
+	"github.com/sipeed/picoclaw/pkg/bus"
+	"github.com/sipeed/picoclaw/pkg/identity"
+	"github.com/sipeed/picoclaw/pkg/logger"
+)
+
+// onConnect is called after a successful connection (and on reconnect).
+func (c *IRCChannel) onConnect(conn *ircevent.Connection) {
+	// NickServ auth (only if SASL is not configured)
+	if c.config.NickServPassword != "" && c.config.SASLUser == "" {
+		conn.Privmsg("NickServ", "IDENTIFY "+c.config.NickServPassword)
+	}
+
+	// Join configured channels
+	for _, ch := range c.config.Channels {
+		conn.Join(ch)
+		logger.InfoCF("irc", "Joined IRC channel", map[string]any{
+			"channel": ch,
+		})
+	}
+}
+
+// onPrivmsg handles incoming PRIVMSG events.
+func (c *IRCChannel) onPrivmsg(conn *ircevent.Connection, e ircmsg.Message) {
+	if len(e.Params) < 2 {
+		return
+	}
+
+	nick := e.Nick()
+	currentNick := conn.CurrentNick()
+
+	// Ignore own messages
+	if strings.EqualFold(nick, currentNick) {
+		return
+	}
+
+	target := e.Params[0]  // channel name or bot's nick
+	content := e.Params[1] // message text
+
+	// Determine if this is a DM or channel message
+	isDM := !strings.HasPrefix(target, "#") && !strings.HasPrefix(target, "&")
+
+	var chatID string
+	var peer bus.Peer
+
+	if isDM {
+		chatID = nick
+		peer = bus.Peer{Kind: "direct", ID: nick}
+	} else {
+		chatID = target
+		peer = bus.Peer{Kind: "group", ID: target}
+	}
+
+	sender := bus.SenderInfo{
+		Platform:    "irc",
+		PlatformID:  nick,
+		CanonicalID: identity.BuildCanonicalID("irc", nick),
+		Username:    nick,
+		DisplayName: nick,
+	}
+
+	if !c.IsAllowedSender(sender) {
+		return
+	}
+
+	// For channel messages, check group trigger (mention detection)
+	if !isDM {
+		isMentioned := isBotMentioned(content, currentNick)
+		if isMentioned {
+			content = stripBotMention(content, currentNick)
+		}
+		respond, cleaned := c.ShouldRespondInGroup(isMentioned, content)
+		if !respond {
+			return
+		}
+		content = cleaned
+	}
+
+	if strings.TrimSpace(content) == "" {
+		return
+	}
+
+	messageID := fmt.Sprintf("%s-%d", nick, time.Now().UnixNano())
+
+	metadata := map[string]string{
+		"platform": "irc",
+		"server":   c.config.Server,
+	}
+	if !isDM {
+		metadata["channel"] = target
+	}
+
+	c.HandleMessage(c.ctx, peer, messageID, nick, chatID, content, nil, metadata, sender)
+}
+
+// isBotMentioned checks if the bot's nick appears in the message.
+func isBotMentioned(content, botNick string) bool {
+	lower := strings.ToLower(content)
+	lowerNick := strings.ToLower(botNick)
+
+	// "nick: " or "nick, " at start (most common IRC convention)
+	if strings.HasPrefix(lower, lowerNick+":") || strings.HasPrefix(lower, lowerNick+",") {
+		return true
+	}
+
+	// Word-boundary match anywhere in the message
+	idx := strings.Index(lower, lowerNick)
+	if idx < 0 {
+		return false
+	}
+	before := idx == 0 || !isAlphanumeric(lower[idx-1])
+	after := idx+len(lowerNick) >= len(lower) || !isAlphanumeric(lower[idx+len(lowerNick)])
+	return before && after
+}
+
+// stripBotMention removes "nick: " or "nick, " prefix from content.
+func stripBotMention(content, botNick string) string {
+	lower := strings.ToLower(content)
+	lowerNick := strings.ToLower(botNick)
+	for _, sep := range []string{":", ","} {
+		prefix := lowerNick + sep
+		if strings.HasPrefix(lower, prefix) {
+			return strings.TrimSpace(content[len(prefix):])
+		}
+	}
+	return content
+}
+
+func isAlphanumeric(b byte) bool {
+	return (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9') || b == '_'
+}