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/ralph-loop/index.ts

@@ -2,6 +2,7 @@ import type { PluginInput } from "@opencode-ai/plugin"
 import { existsSync, readFileSync, readdirSync } from "node:fs"
 import { join } from "node:path"
 import { log } from "../../shared/logger"
+import { SYSTEM_DIRECTIVE_PREFIX } from "../../shared/system-directive"
 import { readState, writeState, clearState, incrementIteration } from "./storage"
 import {
   HOOK_NAME,
@@ -42,7 +43,7 @@ interface OpenCodeSessionMessage {
   }>
 }
 
-const CONTINUATION_PROMPT = `[RALPH LOOP - ITERATION {{ITERATION}}/{{MAX}}]
+const CONTINUATION_PROMPT = `${SYSTEM_DIRECTIVE_PREFIX} - RALPH LOOP {{ITERATION}}/{{MAX}}]
 
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