refactor: unify system directive prefix for keyword-detector filtering

- Add shared/system-directive.ts with SYSTEM_DIRECTIVE_PREFIX constant
- Unify all system message prefixes to [SYSTEM DIRECTIVE: OH-MY-OPENCODE - ...]
- Add isSystemDirective() filter to keyword-detector to skip system messages
- Update prometheus-md-only tests to use new prefix constants

src/hooks/prometheus-md-only/constants.ts

@@ -1,3 +1,5 @@
+import { createSystemDirective, SystemDirectiveTypes } from "../../shared/system-directive"
+
 export const HOOK_NAME = "prometheus-md-only"
 
 export const PROMETHEUS_AGENTS = ["Prometheus (Planner)"]
@@ -12,7 +14,7 @@ export const PLANNING_CONSULT_WARNING = `
 
 ---
 
-[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]
+${createSystemDirective(SystemDirectiveTypes.PROMETHEUS_READ_ONLY)}
 
 You are being invoked by Prometheus (Planner), a READ-ONLY planning agent.