fix: use configured endSymbols in injected prompt and exempt gateway keywords

- buildEndMarkerInstruction() replaces hardcoded END_MARKER_INSTRUCTION,
  dynamically using the resolved policy's endSymbols
- Instruction now explicitly exempts gateway keywords (NO_REPLY, HEARTBEAT_OK)
  from requiring end symbols
- Export resolvePolicy from rules.ts for reuse in before_prompt_build hook

plugin/rules.ts

@@ -46,7 +46,7 @@ function getLastChar(input: string): string {
   return t.length ? t[t.length - 1] : "";
 }
 
-function resolvePolicy(config: WhisperGateConfig, channelId?: string, channelPolicies?: Record<string, ChannelPolicy>) {
+export function resolvePolicy(config: WhisperGateConfig, channelId?: string, channelPolicies?: Record<string, ChannelPolicy>) {
   const globalMode = config.listMode || "human-list";
   const globalHuman = config.humanList || config.bypassUserIds || [];
   const globalAgent = config.agentList || [];