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/compaction-context-injector/index.ts

@@ -1,5 +1,6 @@
 import { injectHookMessage } from "../../features/hook-message-injector"
 import { log } from "../../shared/logger"
+import { createSystemDirective, SystemDirectiveTypes } from "../../shared/system-directive"
 
 export interface SummarizeContext {
   sessionID: string
@@ -9,7 +10,7 @@ export interface SummarizeContext {
   directory: string
 }
 
-const SUMMARIZE_CONTEXT_PROMPT = `[COMPACTION CONTEXT INJECTION]
+const SUMMARIZE_CONTEXT_PROMPT = `${createSystemDirective(SystemDirectiveTypes.COMPACTION_CONTEXT)}
 
 When summarizing this session, you MUST include the following sections in your summary: