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/routing/session_key.go

@@ -163,6 +163,15 @@ func resolveLinkedPeerID(identityLinks map[string][]string, channel, peerID stri
 		scopedCandidate := fmt.Sprintf("%s:%s", channel, strings.ToLower(peerID))
 		candidates[scopedCandidate] = true
 	}
+
+	// If peerID is already in canonical "platform:id" format, also add the
+	// bare ID part as a candidate for backward compatibility with identity_links
+	// that use raw IDs (e.g. "123" instead of "telegram:123").
+	if idx := strings.Index(rawCandidate, ":"); idx > 0 && idx < len(rawCandidate)-1 {
+		bareID := rawCandidate[idx+1:]
+		candidates[bareID] = true
+	}
+
 	if len(candidates) == 0 {
 		return ""
 	}

pkg/routing/session_key_test.go

@@ -115,6 +115,51 @@ func TestBuildAgentPeerSessionKey_IdentityLink(t *testing.T) {
 	}
 }
 
+func TestResolveLinkedPeerID_CanonicalPeerID(t *testing.T) {
+	// When peerID is already in canonical "platform:id" format,
+	// it should match identity_links that use the bare ID.
+	links := map[string][]string{
+		"john": {"123"},
+	}
+	got := resolveLinkedPeerID(links, "telegram", "telegram:123")
+	if got != "john" {
+		t.Errorf("resolveLinkedPeerID with canonical peerID = %q, want %q", got, "john")
+	}
+}
+
+func TestResolveLinkedPeerID_CanonicalInLinks(t *testing.T) {
+	// When identity_links contain canonical IDs and peerID is canonical too
+	links := map[string][]string{
+		"john": {"telegram:123", "discord:456"},
+	}
+	got := resolveLinkedPeerID(links, "telegram", "telegram:123")
+	if got != "john" {
+		t.Errorf("resolveLinkedPeerID canonical in links = %q, want %q", got, "john")
+	}
+}
+
+func TestResolveLinkedPeerID_BarePeerIDMatchesCanonicalLink(t *testing.T) {
+	// When peerID is bare "123" and links have "telegram:123",
+	// the scoped candidate "telegram:123" should match.
+	links := map[string][]string{
+		"john": {"telegram:123"},
+	}
+	got := resolveLinkedPeerID(links, "telegram", "123")
+	if got != "john" {
+		t.Errorf("resolveLinkedPeerID bare peer matches canonical link = %q, want %q", got, "john")
+	}
+}
+
+func TestResolveLinkedPeerID_NoMatch(t *testing.T) {
+	links := map[string][]string{
+		"john": {"telegram:123"},
+	}
+	got := resolveLinkedPeerID(links, "discord", "999")
+	if got != "" {
+		t.Errorf("resolveLinkedPeerID no match = %q, want empty", got)
+	}
+}
+
 func TestParseAgentSessionKey_Valid(t *testing.T) {
 	parsed := ParseAgentSessionKey("agent:sales:telegram:direct:user123")
 	if parsed == nil {