fix: correct $ARGUMENTS token in plugin-compaction-init template

OpenCode's argument token is $ARGUMENTS, not $ARGUMENTS$. With the
extra dollar sign, OpenCode did not substitute the token, so the LLM
ended up writing the literal $ARGUMENTS$ line into the memory file
when no arguments were passed.

src/index.ts

@@ -21,7 +21,7 @@ const initCommandName = "plugin-compaction-init";
 
 /**
  * Builds the prompt the auto-registered `/plugin-compaction-init` slash
- * command sends to the model. `$ARGUMENTS$` is left unsubstituted so
+ * command sends to the model. `$ARGUMENTS` is left unsubstituted so
  * OpenCode substitutes it at invocation time.
  *
  * @param memoryFile Literal path to the memory file, inserted verbatim.
@@ -46,7 +46,7 @@ Keep all details related to the keep topics below, and discard everything relate
 
 - Any other older discussions not mentioned above.
 
-$ARGUMENTS$
+$ARGUMENTS
 `;
 }