feat: big ahh commit

- agent+proto+backend: transfer service status
- agent: fix returning empty message on nonzero exit status
- backend: refactor collector+commander and handlers dependent on them: implement agent accounting via grpc stats handler

backend/internal/grpcsrv/collector/tracker.go

@@ -0,0 +1,111 @@
+package collector
+
+import (
+	"context"
+	"log"
+	"sync"
+	"time"
+
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/stats"
+)
+
+// ConnTracker tracks connected Collector agents and handles cleanup on disconnect.
+// It implements grpc.StatsHandler for disconnect detection.
+type ConnTracker struct {
+	mu     sync.RWMutex
+	agents map[string]*Agent
+}
+
+func NewConnTracker() *ConnTracker {
+	return &ConnTracker{
+		agents: make(map[string]*Agent),
+	}
+}
+
+// Register adds an agent to the tracker. Called by Collector.Stream().
+func (t *ConnTracker) Register(agent *Agent) {
+	t.mu.Lock()
+	t.agents[agent.ID] = agent
+	t.mu.Unlock()
+	log.Printf("[collector] agent registered: %s", agent.ID)
+}
+
+// Unregister removes an agent from the tracker.
+func (t *ConnTracker) Unregister(id string) {
+	t.mu.Lock()
+	delete(t.agents, id)
+	t.mu.Unlock()
+	log.Printf("[collector] agent unregistered: %s", id)
+}
+
+// GetAgent returns the agent for the given ID.
+func (t *ConnTracker) GetAgent(id string) (*Agent, bool) {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	a, ok := t.agents[id]
+	return a, ok
+}
+
+// Agents returns all connected agents.
+func (t *ConnTracker) Agents() []*Agent {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	result := make([]*Agent, 0, len(t.agents))
+	for _, a := range t.agents {
+		result = append(result, a)
+	}
+	return result
+}
+
+// grpc.StatsHandler implementation.
+
+func (t *ConnTracker) TagRPC(ctx context.Context, _ *stats.RPCTagInfo) context.Context {
+	return ctx
+}
+
+func (t *ConnTracker) HandleRPC(ctx context.Context, _ stats.RPCStats) {}
+
+func (t *ConnTracker) TagConn(ctx context.Context, _ *stats.ConnTagInfo) context.Context {
+	return ctx
+}
+
+func (t *ConnTracker) HandleConn(ctx context.Context, s stats.ConnStats) {
+	switch s.(type) {
+	case *stats.ConnEnd:
+		md, ok := metadata.FromIncomingContext(ctx)
+		if !ok {
+			return
+		}
+		whoamiVals := md["whoami"]
+		if len(whoamiVals) == 0 {
+			return
+		}
+		t.Unregister(whoamiVals[0])
+	}
+}
+
+// UpdateServices updates the services list for the given agent.
+func (t *ConnTracker) UpdateServices(id string, services []Service) bool {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	agent, ok := t.agents[id]
+	if !ok {
+		return false
+	}
+	agent.Services = services
+	return true
+}
+
+// Service represents a named service with its current status.
+type Service struct {
+	Name, Status string
+}
+
+// Agent represents a connected agent streaming logs to the collector.
+type Agent struct {
+	ID          string
+	Label       string
+	Services    []Service
+	ConnectedAt time.Time
+}