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/heartbeat.go

@@ -44,17 +44,31 @@ type HeartbeatClient struct {
 // NewHeartbeatClient creates a client that registers with the master and
 // sends periodic heartbeats. Returns nil if master address is not configured.
 func NewHeartbeatClient(cfg config.AgentConfig, logger interface{ Info(string, ...any); Warn(string, ...any); Error(string, ...any) }) (*HeartbeatClient, error) {
-	masterAddr := os.Getenv("MCP_MASTER_ADDRESS")
-	masterCACert := os.Getenv("MCP_MASTER_CA_CERT")
-	masterToken := os.Getenv("MCP_MASTER_TOKEN_PATH")
+	// Config takes precedence, env vars as fallback.
+	masterAddr := cfg.Master.Address
+	if masterAddr == "" {
+		masterAddr = os.Getenv("MCP_MASTER_ADDRESS")
+	}
+	masterCACert := cfg.Master.CACert
+	if masterCACert == "" {
+		masterCACert = os.Getenv("MCP_MASTER_CA_CERT")
+	}
+	masterTokenPath := cfg.Master.TokenPath
+	if masterTokenPath == "" {
+		masterTokenPath = os.Getenv("MCP_MASTER_TOKEN_PATH")
+	}
+	role := cfg.Master.Role
+	if role == "" {
+		role = "worker"
+	}
 
 	if masterAddr == "" {
 		return nil, nil // master not configured
 	}
 
 	token := ""
-	if masterToken != "" {
-		data, err := os.ReadFile(masterToken) //nolint:gosec // trusted config
+	if masterTokenPath != "" {
+		data, err := os.ReadFile(masterTokenPath) //nolint:gosec // trusted config
 		if err != nil {
 			return nil, fmt.Errorf("read master token: %w", err)
 		}
@@ -92,7 +106,7 @@ func NewHeartbeatClient(cfg config.AgentConfig, logger interface{ Info(string, .
 		client:   mcpv1.NewMcpMasterServiceClient(conn),
 		conn:     conn,
 		nodeName: cfg.Agent.NodeName,
-		role:     "worker", // default; master node sets this via config
+		role:     role,
 		address:  cfg.Server.GRPCAddr,
 		arch:     runtime.GOARCH,
 		interval: 30 * time.Second,