feat(plugin): inject 🔚 ending instruction for bypass/end-symbol discord turns

docs/IMPLEMENTATION.md

@@ -9,6 +9,10 @@ WhisperGate evaluates in strict order:
 3. message ending symbol check
 4. fallback to no-reply model override
 
+Additional prompt behavior:
+- when decision is `bypass_sender` or `end_symbol:*`, plugin prepends:
+  - `你的这次发言必须以🔚作为结尾。`
+
 ## Why before_model_resolve
 
 - deterministic