feat(channels): add Slack channel integration with Socket Mode

Add Slack as a messaging channel using Socket Mode (WebSocket), bringing
the total supported channels to 8. Features include bidirectional
messaging, thread support with per-thread session context, @mention
handling, ack reactions (👀/✅), slash commands,
file/attachment support with Groq Whisper audio transcription, and
allowlist filtering by Slack user ID.

Closes #31

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/channels/manager.go

@@ -136,6 +136,19 @@ func (m *Manager) initChannels() error {
 		}
 	}
 
+	if m.config.Channels.Slack.Enabled && m.config.Channels.Slack.BotToken != "" {
+		logger.DebugC("channels", "Attempting to initialize Slack channel")
+		slackCh, err := NewSlackChannel(m.config.Channels.Slack, m.bus)
+		if err != nil {
+			logger.ErrorCF("channels", "Failed to initialize Slack channel", map[string]interface{}{
+				"error": err.Error(),
+			})
+		} else {
+			m.channels["slack"] = slackCh
+			logger.InfoC("channels", "Slack channel enabled successfully")
+		}
+	}
+
 	logger.InfoCF("channels", "Channel initialization completed", map[string]interface{}{
 		"enabled_channels": len(m.channels),
 	})