fix(cli): replace dead-end \mem0 link\ tip in claim flow

The "email_already_claimed" tip in \`claim_via_otp\` (Python + Node)
referenced a \`mem0 link <key>\` command that doesn't exist. Real users
hit this dead end when they had a dashboard account predating their
agent's signup.

Replace with honest copy: "Sign in at app.mem0.ai with your existing
credentials." No promise of merging — that flow isn't built yet
(deferred design in platform repo .plans/).

Folded into this docs-promo PR rather than a separate small CLI PR
since it's user-facing wording.

Companion platform fix: mem0ai/platform#2790 (backend error messages
that suggested the same non-existent command).

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);

cli/python/src/mem0_cli/commands/agent_mode_cmd.py

@@ -218,7 +218,7 @@ def claim_via_otp(config: Mem0Config, *, email: str, code: str | None = None) ->
         print_error(err_console, f"Claim failed: {detail}")
         if code_str == "email_already_claimed":
             console.print(
-                f"  [{DIM_COLOR}]Tip: this email already has a Mem0 account. Sign in there and run `mem0 link <key>` to attach this agent.[/]"
+                f"  [{DIM_COLOR}]Tip: this email already has a Mem0 account. Sign in at app.mem0.ai with your existing credentials.[/]"
             )
         raise typer.Exit(1)