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/slack_test.go

@@ -0,0 +1,193 @@
+package channels
+
+import (
+	"testing"
+
+	"github.com/sipeed/picoclaw/pkg/bus"
+	"github.com/sipeed/picoclaw/pkg/config"
+)
+
+func TestParseSlackChatID(t *testing.T) {
+	tests := []struct {
+		name       string
+		chatID     string
+		wantChanID string
+		wantThread string
+	}{
+		{
+			name:       "channel only",
+			chatID:     "C123456",
+			wantChanID: "C123456",
+			wantThread: "",
+		},
+		{
+			name:       "channel with thread",
+			chatID:     "C123456/1234567890.123456",
+			wantChanID: "C123456",
+			wantThread: "1234567890.123456",
+		},
+		{
+			name:       "DM channel",
+			chatID:     "D987654",
+			wantChanID: "D987654",
+			wantThread: "",
+		},
+		{
+			name:       "empty string",
+			chatID:     "",
+			wantChanID: "",
+			wantThread: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			chanID, threadTS := parseSlackChatID(tt.chatID)
+			if chanID != tt.wantChanID {
+				t.Errorf("parseSlackChatID(%q) channelID = %q, want %q", tt.chatID, chanID, tt.wantChanID)
+			}
+			if threadTS != tt.wantThread {
+				t.Errorf("parseSlackChatID(%q) threadTS = %q, want %q", tt.chatID, threadTS, tt.wantThread)
+			}
+		})
+	}
+}
+
+func TestStripBotMention(t *testing.T) {
+	ch := &SlackChannel{botUserID: "U12345BOT"}
+
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			name:  "mention at start",
+			input: "<@U12345BOT> hello there",
+			want:  "hello there",
+		},
+		{
+			name:  "mention in middle",
+			input: "hey <@U12345BOT> can you help",
+			want:  "hey  can you help",
+		},
+		{
+			name:  "no mention",
+			input: "hello world",
+			want:  "hello world",
+		},
+		{
+			name:  "empty string",
+			input: "",
+			want:  "",
+		},
+		{
+			name:  "only mention",
+			input: "<@U12345BOT>",
+			want:  "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ch.stripBotMention(tt.input)
+			if got != tt.want {
+				t.Errorf("stripBotMention(%q) = %q, want %q", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestNewSlackChannel(t *testing.T) {
+	msgBus := bus.NewMessageBus()
+
+	t.Run("missing bot token", func(t *testing.T) {
+		cfg := config.SlackConfig{
+			BotToken: "",
+			AppToken: "xapp-test",
+		}
+		_, err := NewSlackChannel(cfg, msgBus)
+		if err == nil {
+			t.Error("expected error for missing bot_token, got nil")
+		}
+	})
+
+	t.Run("missing app token", func(t *testing.T) {
+		cfg := config.SlackConfig{
+			BotToken: "xoxb-test",
+			AppToken: "",
+		}
+		_, err := NewSlackChannel(cfg, msgBus)
+		if err == nil {
+			t.Error("expected error for missing app_token, got nil")
+		}
+	})
+
+	t.Run("valid config", func(t *testing.T) {
+		cfg := config.SlackConfig{
+			BotToken:  "xoxb-test",
+			AppToken:  "xapp-test",
+			AllowFrom: []string{"U123"},
+		}
+		ch, err := NewSlackChannel(cfg, msgBus)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if ch.Name() != "slack" {
+			t.Errorf("Name() = %q, want %q", ch.Name(), "slack")
+		}
+		if ch.IsRunning() {
+			t.Error("new channel should not be running")
+		}
+	})
+}
+
+func TestSlackChannelIsAllowed(t *testing.T) {
+	msgBus := bus.NewMessageBus()
+
+	t.Run("empty allowlist allows all", func(t *testing.T) {
+		cfg := config.SlackConfig{
+			BotToken:  "xoxb-test",
+			AppToken:  "xapp-test",
+			AllowFrom: []string{},
+		}
+		ch, _ := NewSlackChannel(cfg, msgBus)
+		if !ch.IsAllowed("U_ANYONE") {
+			t.Error("empty allowlist should allow all users")
+		}
+	})
+
+	t.Run("allowlist restricts users", func(t *testing.T) {
+		cfg := config.SlackConfig{
+			BotToken:  "xoxb-test",
+			AppToken:  "xapp-test",
+			AllowFrom: []string{"U_ALLOWED"},
+		}
+		ch, _ := NewSlackChannel(cfg, msgBus)
+		if !ch.IsAllowed("U_ALLOWED") {
+			t.Error("allowed user should pass allowlist check")
+		}
+		if ch.IsAllowed("U_BLOCKED") {
+			t.Error("non-allowed user should be blocked")
+		}
+	})
+}
+
+func TestTruncateStringSlack(t *testing.T) {
+	tests := []struct {
+		input  string
+		maxLen int
+		want   string
+	}{
+		{"hello", 10, "hello"},
+		{"hello world", 5, "hello"},
+		{"", 5, ""},
+	}
+
+	for _, tt := range tests {
+		got := truncateStringSlack(tt.input, tt.maxLen)
+		if got != tt.want {
+			t.Errorf("truncateStringSlack(%q, %d) = %q, want %q", tt.input, tt.maxLen, got, tt.want)
+		}
+	}
+}