Introduce SenderInfo struct and pkg/identity package to standardize user
identification across all channels. Each channel now constructs structured
sender info (platform, platformID, canonicalID, username, displayName)
instead of ad-hoc string IDs. Allow-list matching supports all legacy
formats (numeric ID, @username, id|username) plus the new canonical
"platform:id" format. Session key resolution also handles canonical
peerIDs for backward-compatible identity link matching.
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.
Hoshina