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>

cmd/picoclaw/main.go

@@ -586,6 +586,12 @@ func gatewayCmd() {
 				logger.InfoC("voice", "Groq transcription attached to Discord channel")
 			}
 		}
+		if slackChannel, ok := channelManager.GetChannel("slack"); ok {
+			if sc, ok := slackChannel.(*channels.SlackChannel); ok {
+				sc.SetTranscriber(transcriber)
+				logger.InfoC("voice", "Groq transcription attached to Slack channel")
+			}
+		}
 	}
 
 	enabledChannels := channelManager.GetEnabledChannels()