feat(identity): add unified user identity with canonical platform:id format

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.

pkg/bus/types.go

@@ -6,9 +6,19 @@ type Peer struct {
 	ID   string `json:"id"`
 }
 
+// SenderInfo provides structured sender identity information.
+type SenderInfo struct {
+	Platform    string `json:"platform,omitempty"`     // "telegram", "discord", "slack", ...
+	PlatformID  string `json:"platform_id,omitempty"`  // raw platform ID, e.g. "123456"
+	CanonicalID string `json:"canonical_id,omitempty"` // "platform:id" format
+	Username    string `json:"username,omitempty"`     // username (e.g. @alice)
+	DisplayName string `json:"display_name,omitempty"` // display name
+}
+
 type InboundMessage struct {
 	Channel    string            `json:"channel"`
 	SenderID   string            `json:"sender_id"`
+	Sender     SenderInfo        `json:"sender"`
 	ChatID     string            `json:"chat_id"`
 	Content    string            `json:"content"`
 	Media      []string          `json:"media,omitempty"`