refactor(session): replace dm scope with dimensions policy

pkg/routing/route.go

@@ -17,10 +17,8 @@ type RouteInput struct {
 }
 
 // SessionPolicy describes how a routed message should be mapped to a session.
-// The current implementation preserves the legacy dm_scope and identity_link
-// semantics while moving session-key construction out of the router.
 type SessionPolicy struct {
-	DMScope       DMScope
+	Dimensions    []string
 	IdentityLinks map[string][]string
 }
 
@@ -246,16 +244,38 @@ func (r *RouteResolver) resolveDefaultAgentID() string {
 }
 
 func (r *RouteResolver) sessionPolicy() SessionPolicy {
-	dmScope := DMScope(r.cfg.Session.DMScope)
-	if dmScope == "" {
-		dmScope = DMScopeMain
-	}
 	return SessionPolicy{
-		DMScope:       dmScope,
+		Dimensions:    normalizeSessionDimensions(r.cfg.Session.Dimensions),
 		IdentityLinks: cloneIdentityLinks(r.cfg.Session.IdentityLinks),
 	}
 }
 
+func normalizeSessionDimensions(dimensions []string) []string {
+	if len(dimensions) == 0 {
+		return nil
+	}
+
+	normalized := make([]string, 0, len(dimensions))
+	seen := make(map[string]struct{}, len(dimensions))
+	for _, dimension := range dimensions {
+		dimension = strings.ToLower(strings.TrimSpace(dimension))
+		switch dimension {
+		case "space", "chat", "topic", "sender":
+		default:
+			continue
+		}
+		if _, ok := seen[dimension]; ok {
+			continue
+		}
+		seen[dimension] = struct{}{}
+		normalized = append(normalized, dimension)
+	}
+	if len(normalized) == 0 {
+		return nil
+	}
+	return normalized
+}
+
 func cloneIdentityLinks(src map[string][]string) map[string][]string {
 	if len(src) == 0 {
 		return nil

pkg/routing/route_test.go

@@ -17,7 +17,7 @@ func testConfig(agents []config.AgentConfig, bindings []config.AgentBinding) *co
 		},
 		Bindings: bindings,
 		Session: config.SessionConfig{
-			DMScope: "per-peer",
+			Dimensions: []string{"sender"},
 		},
 	}
 }
@@ -37,8 +37,8 @@ func TestResolveRoute_DefaultAgent_NoBindings(t *testing.T) {
 	if route.MatchedBy != "default" {
 		t.Errorf("MatchedBy = %q, want 'default'", route.MatchedBy)
 	}
-	if route.SessionPolicy.DMScope != DMScopePerPeer {
-		t.Errorf("SessionPolicy.DMScope = %q, want %q", route.SessionPolicy.DMScope, DMScopePerPeer)
+	if len(route.SessionPolicy.Dimensions) != 1 || route.SessionPolicy.Dimensions[0] != "sender" {
+		t.Errorf("SessionPolicy.Dimensions = %v, want [sender]", route.SessionPolicy.Dimensions)
 	}
 	if route.SessionPolicy.IdentityLinks != nil {
 		t.Errorf("SessionPolicy.IdentityLinks = %v, want nil", route.SessionPolicy.IdentityLinks)

pkg/routing/session_key.go

@@ -112,6 +112,19 @@ func CanonicalSessionPeerID(
 	return strings.ToLower(normalizedPeerID)
 }
 
+// CanonicalSessionIdentityID collapses an identity using identity_links when
+// possible, then returns a normalized lowercase identifier.
+func CanonicalSessionIdentityID(channel, rawID string, identityLinks map[string][]string) string {
+	normalizedID := strings.TrimSpace(rawID)
+	if normalizedID == "" {
+		return ""
+	}
+	if linked := resolveLinkedPeerID(identityLinks, channel, normalizedID); linked != "" {
+		normalizedID = linked
+	}
+	return strings.ToLower(normalizedID)
+}
+
 // ParseAgentSessionKey extracts agentId and rest from "agent:<agentId>:<rest>".
 func ParseAgentSessionKey(sessionKey string) *ParsedSessionKey {
 	raw := strings.TrimSpace(sessionKey)