session: expose Agent() accessor, add methods to Agent struct

- Session.Agent() returns the active *Agent
- Agent.Name(), Agent.BackendName(), Agent.ModelName() — first-class
- Session proxy methods delegate to Agent

Consumers can now access the agent directly:
  sess.Agent().Name()
  sess.Agent().BackendName()

session/agent.go

@@ -28,9 +28,28 @@ type Agent struct {
 }
 
 // Backend returns the active backend from the runtime.
-func (r *Agent) Backend() backend.Backend {
-	if r.runtime == nil {
+func (ag *Agent) Backend() backend.Backend {
+	if ag.runtime == nil {
 		return nil
 	}
-	return r.runtime.Backend
+	return ag.runtime.Backend
+}
+
+// Name returns the agent's name.
+func (ag *Agent) Name() string { return ag.agentName }
+
+// BackendName returns the name of the active backend.
+func (ag *Agent) BackendName() string {
+	if ag.runtime == nil || ag.runtime.Backend == nil {
+		return ""
+	}
+	return ag.runtime.Backend.Name()
+}
+
+// ModelName returns the name of the active model.
+func (ag *Agent) ModelName() string {
+	if ag.runtime == nil || ag.runtime.Backend == nil {
+		return ""
+	}
+	return ag.runtime.Backend.Model()
 }

session/session.go

@@ -703,21 +703,24 @@ func (a *Session) ReactTo(responseID, emoji string) error {
 }
 
 func (a *Session) AgentName() string {
-	v := a.r.agentName
+	v := a.r.Name()
 	a.log.Debug("AgentName() = %q", v)
 	return v
 }
 func (a *Session) BackendName() string {
-	v := a.r.runtime.Backend.Name()
+	v := a.r.BackendName()
 	a.log.Debug("BackendName() = %q", v)
 	return v
 }
 func (a *Session) ModelName() string {
-	v := a.r.runtime.Backend.Model()
+	v := a.r.ModelName()
 	a.log.Debug("ModelName() = %q", v)
 	return v
 }
 
+// Agent returns the active agent.
+func (a *Session) Agent() *Agent { return a.r }
+
 func (a *Session) State() string {
 	return a.state
 }