Add [master] config section to agent for registration

Heartbeat client now reads master connection settings from the agent
config ([master] section) with env var fallback. Includes address,
ca_cert, token_path, and role fields.

Agent's Run() creates and starts the heartbeat client automatically
when [master] is configured.

Tested on all three nodes: rift (master), svc (edge), orion (worker)
all registered with the master and sending heartbeats every 30s.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/agent/agent.go

@@ -131,6 +131,15 @@ func Run(cfg *config.AgentConfig, version string) error {
 		}
 	}
 
+	// Start heartbeat client (registers with master and sends heartbeats).
+	hbClient, hbErr := NewHeartbeatClient(*cfg, logger)
+	if hbErr != nil {
+		logger.Warn("heartbeat client failed to start", "err", hbErr)
+	} else if hbClient != nil {
+		hbClient.Start()
+		logger.Info("heartbeat client started", "master", cfg.Master.Address)
+	}
+
 	mon.Start()
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
@@ -144,6 +153,9 @@ func Run(cfg *config.AgentConfig, version string) error {
 	select {
 	case <-ctx.Done():
 		logger.Info("shutting down")
+		if hbClient != nil {
+			hbClient.Stop()
+		}
 		mon.Stop()
 		server.GracefulStop()
 		_ = proxy.Close()