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/config/agent.go

@@ -19,7 +19,17 @@ type AgentConfig struct {
 	MCNS      MCNSConfig      `toml:"mcns"`
 	Monitor   MonitorConfig   `toml:"monitor"`
 	Log       LogConfig       `toml:"log"`
-	Boot      BootConfig      `toml:"boot"`
+	Boot   BootConfig        `toml:"boot"`
+	Master AgentMasterConfig `toml:"master"`
+}
+
+// AgentMasterConfig holds the optional master connection settings.
+// When configured, the agent self-registers and sends heartbeats.
+type AgentMasterConfig struct {
+	Address   string `toml:"address"`    // master gRPC address
+	CACert    string `toml:"ca_cert"`    // CA cert to verify master's TLS
+	TokenPath string `toml:"token_path"` // MCIAS service token for auth
+	Role      string `toml:"role"`       // "worker", "edge", or "master"
 }
 
 // BootConfig holds the boot sequence for the master node.