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/config/config.go

@@ -38,6 +38,7 @@ type ChannelsConfig struct {
 	MaixCam  MaixCamConfig  `json:"maixcam"`
 	QQ       QQConfig       `json:"qq"`
 	DingTalk DingTalkConfig `json:"dingtalk"`
+	Slack    SlackConfig    `json:"slack"`
 }
 
 type WhatsAppConfig struct {
@@ -88,6 +89,13 @@ type DingTalkConfig struct {
 	AllowFrom        []string `json:"allow_from" env:"PICOCLAW_CHANNELS_DINGTALK_ALLOW_FROM"`
 }
 
+type SlackConfig struct {
+	Enabled  bool     `json:"enabled" env:"PICOCLAW_CHANNELS_SLACK_ENABLED"`
+	BotToken string   `json:"bot_token" env:"PICOCLAW_CHANNELS_SLACK_BOT_TOKEN"`
+	AppToken string   `json:"app_token" env:"PICOCLAW_CHANNELS_SLACK_APP_TOKEN"`
+	AllowFrom []string `json:"allow_from" env:"PICOCLAW_CHANNELS_SLACK_ALLOW_FROM"`
+}
+
 type ProvidersConfig struct {
 	Anthropic  ProviderConfig `json:"anthropic"`
 	OpenAI     ProviderConfig `json:"openai"`
@@ -174,6 +182,12 @@ func DefaultConfig() *Config {
 				ClientSecret: "",
 				AllowFrom:    []string{},
 			},
+			Slack: SlackConfig{
+				Enabled:   false,
+				BotToken:  "",
+				AppToken:  "",
+				AllowFrom: []string{},
+			},
 		},
 		Providers: ProvidersConfig{
 			Anthropic:  ProviderConfig{},