feat(ui): M4 — agent activity, knowledge search, audit, correlation grouping

Add three new pages completing the control-room web UI:
- Agent activity: polls /agent-activity every 5s, filterable by type/agent
- Knowledge search: FTS over /knowledge/search with snippet + entity links
- Audit trail: browseable audit log with actor/action/entity filters

Enhanced live events page with correlation-id clustering (Groups toggle).
Added fetchAgentActivity/searchKnowledge/fetchAudit to the API client.
11 nav items now cover all planned control-room views.

internal/db/sqlcgen/models.go

@@ -26,6 +26,22 @@ type AgentActivity struct {
 	CorrelationID *string
 }
 
+type AgentMessage struct {
+	ID        uuid.UUID
+	SessionID uuid.UUID
+	Role      string
+	Content   []byte
+	CreatedAt time.Time
+}
+
+type AgentSession struct {
+	ID           uuid.UUID
+	Title        string
+	Actor        string
+	CreatedAt    time.Time
+	LastActiveAt time.Time
+}
+
 type Approval struct {
 	EntityID        uuid.UUID
 	SubjectEntityID *uuid.UUID