feat(sessions): SandboxFleet integration — fleet controller primary, SandboxManager fallback

FleetClient calls tool-definition-controller:8788 /sessions/{id}/assign
as the primary sandbox create path. Falls back to direct k8s SandboxManager
if fleet controller is unreachable.

handleSandboxCreate:
  1. fleet.Assign() → podSvcDns from pre-warmed fleet pod (fast, no wait)
  2. fallback: sandbox.CreateOrResume() → dedicated pod (slow, 30s wait)

handleSandboxDelete: fleet.Release() + sandbox.Delete() both run (non-fatal).
updateSandboxSummary: extracted helper, updates session memory_entity with
  pod_name / svc_dns / last_active_at.

NetworkPolicy: added port 8788 to poimen egress (fleet controller REST API).

Response adds fleet:true field when served by fleet.

internal/sessions/fleet_client.go

@@ -0,0 +1,97 @@
+// Package sessions — FleetClient calls the SandboxFleet controller REST API
+// to assign sessions to fleet pods. This is the primary sandbox path;
+// the raw-k8s SandboxManager is the fallback when the fleet controller
+// is unavailable (e.g. during fleet rollout).
+package sessions
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"time"
+)
+
+const (
+	defaultFleetControllerURL = "http://tool-definition-controller.poimen.svc.cluster.local:8788"
+)
+
+// FleetClient calls the SandboxFleet controller REST API.
+type FleetClient struct {
+	baseURL string
+	client  *http.Client
+}
+
+// FleetAssignResult is returned by the controller assign endpoint.
+type FleetAssignResult struct {
+	PodSvcDns string `json:"podSvcDns"`
+	PodName   string `json:"podName"`
+}
+
+// NewFleetClient creates a FleetClient from env or default URL.
+// FLEET_CONTROLLER_URL — default: defaultFleetControllerURL
+func NewFleetClient() *FleetClient {
+	url := os.Getenv("FLEET_CONTROLLER_URL")
+	if url == "" {
+		url = defaultFleetControllerURL
+	}
+	return &FleetClient{
+		baseURL: url,
+		client:  &http.Client{Timeout: 35 * time.Second},
+	}
+}
+
+// Assign asks the fleet controller to assign sessionID to a fleet pod.
+// Returns the pod's ClusterIP service DNS and pod name.
+func (f *FleetClient) Assign(ctx context.Context, sessionID string) (*FleetAssignResult, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
+		f.baseURL+"/sessions/"+sessionID+"/assign", nil)
+	if err != nil {
+		return nil, fmt.Errorf("build request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := f.client.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("fleet controller unreachable: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= 300 {
+		body, _ := io.ReadAll(io.LimitReader(resp.Body, 512))
+		return nil, fmt.Errorf("fleet controller returned %d: %s", resp.StatusCode, body)
+	}
+
+	var result FleetAssignResult
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		return nil, fmt.Errorf("decode: %w", err)
+	}
+	if result.PodSvcDns == "" {
+		return nil, fmt.Errorf("fleet controller returned empty podSvcDns")
+	}
+	return &result, nil
+}
+
+// Release notifies the fleet controller that a session has ended.
+// Cleans up the per-session ServiceAccount (audit trail SA).
+// Non-fatal: fleet controller handles this async.
+func (f *FleetClient) Release(ctx context.Context, sessionID string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodDelete,
+		f.baseURL+"/sessions/"+sessionID, bytes.NewReader(nil))
+	if err != nil {
+		return fmt.Errorf("build request: %w", err)
+	}
+	resp, err := f.client.Do(req)
+	if err != nil {
+		return fmt.Errorf("fleet controller unreachable: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode >= 300 {
+		body, _ := io.ReadAll(io.LimitReader(resp.Body, 512))
+		return fmt.Errorf("fleet controller returned %d: %s", resp.StatusCode, body)
+	}
+	return nil
+}