feat(api): HTTP API REST+SSE para gestion remota de agentes (issue 0128)

Nuevo paquete internal/api con servidor HTTP stdlib (sin gin/echo):
- Auth Bearer via AGENTS_API_KEY con subtle.ConstantTimeCompare
- REST: GET /health (sin auth), GET/POST /agents, /agents/{id}, /{id}/{start,stop,restart,logs}
- SSE: /sse/status (broadcast diffs cada 2s) y /sse/agents/{id}/logs (tail -f)
- Pubsub in-memory (TODO: NATS cuando haya 2do cliente)
- Tail de logfiles: retroalimenta ultimos 50KB + poll 200ms para streaming

Integracion en cmd/launcher/main.go:
- Flag --api-port (0=desactivado, 8487 en produccion)
- Flag --api-key (override de AGENTS_API_KEY env var)
- Si apiPort>0 y sin clave, WARN y deshabilita en vez de fallar

Systemd unit en systemd/agents_and_robots.service:
- Restart=always (no on-failure — evita que exit limpio mate el service)
- EnvironmentFile para AGENTS_API_KEY y demas tokens
- WorkingDirectory=/home/ubuntu/CodeProyects/agents_and_robots

app.md v0.2.0:
- port: 8487, health_endpoint: /health (fix drift anterior donde era null)
- e2e_checks: build, tests, smoke_health, smoke_auth
- Documentacion Traefik+DNS pendiente humano post-merge

Tests: 12 tests unitarios en internal/api (auth, health, bus, agents, logs)
Smoke: /health 200, /agents sin auth 401, /agents con key 200 — verificado local

Co-Authored-By: fn-constructor (agent)

internal/api/poller.go

@@ -0,0 +1,91 @@
+package api
+
+import (
+	"context"
+	"time"
+
+	"github.com/enmanuel/agents/shell/process"
+)
+
+// StatusDiff is published to the "status" topic when an agent's running state changes.
+type StatusDiff struct {
+	AgentID   string `json:"agent_id"`
+	OldStatus bool   `json:"old_running"`
+	NewStatus bool   `json:"new_running"`
+	PID       int    `json:"pid,omitempty"`
+}
+
+// pollStatus polls StatusAll every 2s and publishes StatusDiff events on changes.
+func (s *Server) pollStatus(ctx context.Context) {
+	ticker := time.NewTicker(2 * time.Second)
+	defer ticker.Stop()
+
+	// Seed the previous state map.
+	prev := make(map[string]bool)
+	if statuses, err := s.mgr.StatusAll(); err == nil {
+		for _, st := range statuses {
+			prev[st.ID] = st.Running
+		}
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			s.checkAndPublishDiffs(prev)
+		}
+	}
+}
+
+func (s *Server) checkAndPublishDiffs(prev map[string]bool) {
+	statuses, err := s.mgr.StatusAll()
+	if err != nil {
+		return
+	}
+
+	for _, st := range statuses {
+		old, known := prev[st.ID]
+		if !known || old != st.Running {
+			s.bus.Publish("status", StatusDiff{
+				AgentID:   st.ID,
+				OldStatus: old,
+				NewStatus: st.Running,
+				PID:       st.PID,
+			})
+			prev[st.ID] = st.Running
+		}
+	}
+
+	// Handle agents that were removed (disappeared from scan)
+	current := make(map[string]bool, len(statuses))
+	for _, st := range statuses {
+		current[st.ID] = true
+	}
+	for id, wasRunning := range prev {
+		if !current[id] {
+			if wasRunning {
+				s.bus.Publish("status", StatusDiff{
+					AgentID:   id,
+					OldStatus: true,
+					NewStatus: false,
+				})
+			}
+			delete(prev, id)
+		}
+	}
+}
+
+// agentInfoByID finds AgentInfo by ID in a StatusAll scan.
+func agentInfoByID(mgr *process.Manager, id string) (*process.AgentInfo, error) {
+	agents, err := mgr.Scan()
+	if err != nil {
+		return nil, err
+	}
+	for i, a := range agents {
+		if a.ID == id {
+			return &agents[i], nil
+		}
+	}
+	return nil, nil
+}