fix: prevent duplicate assistant texts from whitespace differences

- Add per-message dedup tracking in subscribeEmbeddedPiSession
- Compare both trimmed and normalized text to catch near-duplicates
- Reset dedup state on each new assistant message
- Add test for trailing whitespace edge case

Fixes duplicate Slack message delivery when the same text appears
with minor whitespace differences (e.g., trailing newline).

src/agents/pi-embedded-subscribe.handlers.types.ts

@@ -39,6 +39,10 @@ export type EmbeddedPiSubscribeState = {
   lastStreamedAssistant?: string;
   lastStreamedReasoning?: string;
   lastBlockReplyText?: string;
+  assistantMessageIndex: number;
+  lastAssistantTextMessageIndex: number;
+  lastAssistantTextNormalized?: string;
+  lastAssistantTextTrimmed?: string;
   assistantTextBaseline: number;
   suppressBlockChunks: boolean;
   lastReasoningSent?: string;