fix(await-signal): update agent last_activity on signal received (#774)

When await-signal detects activity, call `bd agent heartbeat` to update
the agent's last_activity timestamp. This enables witnesses to accurately
detect agent liveness and prevents false "agent unresponsive" alerts.

Previously, await-signal only managed the idle:N label but never updated
last_activity, causing it to remain NULL for all agents.

Fixes #773

internal/cmd/molecule_await_signal.go

@@ -160,7 +160,14 @@ func runMoleculeAwaitSignal(cmd *cobra.Command, args []string) error {
 			result.IdleCycles = newIdleCycles
 		}
 	} else if result.Reason == "signal" && awaitSignalAgentBead != "" {
-		// On signal, report current idle cycles (caller should reset)
+		// On signal, update last_activity to prove agent is alive
+		if err := updateAgentHeartbeat(awaitSignalAgentBead, beadsDir); err != nil {
+			if !awaitSignalQuiet {
+				fmt.Printf("%s Failed to update agent heartbeat: %v\n",
+					style.Dim.Render("⚠"), err)
+			}
+		}
+		// Report current idle cycles (caller should reset)
 		result.IdleCycles = idleCycles
 	}
 
@@ -319,6 +326,14 @@ func parseIntSimple(s string) (int, error) {
 	return n, nil
 }
 
+// updateAgentHeartbeat updates the last_activity timestamp on an agent bead.
+// This proves the agent is alive and processing signals.
+func updateAgentHeartbeat(agentBead, beadsDir string) error {
+	cmd := exec.Command("bd", "agent", "heartbeat", agentBead)
+	cmd.Env = append(os.Environ(), "BEADS_DIR="+beadsDir)
+	return cmd.Run()
+}
+
 // setAgentIdleCycles sets the idle:N label on an agent bead.
 // Uses read-modify-write pattern to update only the idle label.
 func setAgentIdleCycles(agentBead, beadsDir string, cycles int) error {