docs: promote "Sign up as an agent" + drop plugin-sync prose (#5152)

cli/node/src/commands/agent-mode.ts

@@ -247,7 +247,7 @@ export async function claimViaOtp(
 		printError(`Claim failed: ${detail}`);
 		if (errCode === "email_already_claimed") {
 			console.log(
-				`  ${dim("Tip: this email already has a Mem0 account. Sign in there and run `mem0 link <key>` to attach this agent.")}`,
+				`  ${dim("Tip: this email already has a Mem0 account. Sign in at app.mem0.ai with your existing credentials.")}`,
 			);
 		}
 		process.exit(1);