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/shared/index.ts

@@ -24,3 +24,4 @@ export * from "./zip-extractor"
 export * from "./agent-variant"
 export * from "./session-cursor"
 export * from "./shell-env"
+export * from "./system-directive"