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.

test/index.test.ts

@@ -179,10 +179,10 @@ describe("buildInitCommandTemplate", () => {
     expect(template).toContain("## Discard\n");
   });
 
-  test("leaves $ARGUMENTS$ unsubstituted for OpenCode to fill in", () => {
+  test("leaves $ARGUMENTS unsubstituted for OpenCode to fill in", () => {
     const template = buildInitCommandTemplate(".opencode/compaction.md");
 
-    expect(template.trimEnd().endsWith("$ARGUMENTS$")).toBe(true);
+    expect(template.trimEnd().endsWith("$ARGUMENTS")).toBe(true);
   });
 });
 
@@ -210,7 +210,7 @@ describe("CompactionPromptPlugin config hook", () => {
       "## Discard",
     );
     expect(cfg.command?.["plugin-compaction-init"].template).toContain(
-      "$ARGUMENTS$",
+      "$ARGUMENTS",
     );
     expect(context.logs).toHaveLength(0);
   });