fix: use case-insensitive matching for prometheus agent detection in prometheus-md-only hook

The hook used exact string equality (agentName !== "prometheus") which fails
when display names like "Prometheus (Plan Builder)" are stored in session state.
Replace with case-insensitive substring matching via isPrometheusAgent() helper,
consistent with the pattern used in keyword-detector hook.

Closes #1764 (Bug 3)

src/hooks/prometheus-md-only/agent-matcher.ts

@@ -0,0 +1,5 @@
+import { PROMETHEUS_AGENT } from "./constants"
+
+export function isPrometheusAgent(agentName: string | undefined): boolean {
+  return agentName?.toLowerCase().includes(PROMETHEUS_AGENT) ?? false
+}