refactor(channels): standardize group chat trigger filtering (Phase 8)

Add unified ShouldRespondInGroup to BaseChannel, replacing scattered
per-channel group filtering logic. Introduce GroupTriggerConfig (with
mention_only + prefixes), TypingConfig, and PlaceholderConfig types.
Migrate Discord MentionOnly, OneBot checkGroupTrigger, and LINE
hardcoded mention-only to the shared mechanism. Add group trigger
entry points for Slack, Telegram, QQ, Feishu, DingTalk, and WeCom.
Legacy config fields are preserved with automatic migration.

pkg/config/defaults.go

@@ -80,6 +80,7 @@ func DefaultConfig() *Config {
 				WebhookPort:        18791,
 				WebhookPath:        "/webhook/line",
 				AllowFrom:          FlexibleStringSlice{},
+				GroupTrigger:       GroupTriggerConfig{MentionOnly: true},
 			},
 			OneBot: OneBotConfig{
 				Enabled:            false,