Add scale limit constants, stale-agent indexed query, and tests.

Extract fleet caps (128 hosts, sem=16, 250ms coalesce, syscheck=20) into
named constants with Go tests; wire ListStaleOnlineAgents for stale sweeps
instead of full ListAgents scans.

agent/deploy/scale_limits.go

@@ -0,0 +1,7 @@
+package deploy
+
+// Fleet discovery and lateral spread scale ceilings (per agent).
+const (
+	// SpreadConcurrencyCap limits concurrent spread goroutines per agent.
+	SpreadConcurrencyCap = 16
+)