feat(cli): emit JSON envelope on init --agent --json reuse paths

PRD's documented form is `mem0 init --agent --json`, but rules 1/2
(env/config reuse) called printSuccess which is silenced under
agent mode — so the command exited 0 with no output.

Two related fixes:

1. Add `--json` as a subcommand-level option on init (Node only;
   Python's argv preprocessor already handles this). Lets the
   PRD-style invocation `mem0 init --agent --json` parse without
   "unknown option" error.

2. When agent mode is set AND a rule 1/2 reuse fires, emit the
   Dev Spec C4 envelope:
     {
       "status": "success", "command": "init",
       "data": {
         "api_key_saved": false,
         "api_key_source": "env" | "config",
         "agent_mode": false,
         "message": "Existing Mem0 API key found and reused..."
       }
     }
   Identical shape between Python and Node (parity).

Verified live: `init --agent --json` against prod with a valid
MEM0_API_KEY env returns the envelope above (no bootstrap call).

cli/node/src/commands/init.ts

@@ -21,6 +21,8 @@ import {
 	redactKey,
 	saveConfig,
 } from "../config.js";
+import { formatJsonEnvelope } from "../output.js";
+import { isAgentMode } from "../state.js";
 
 const { brand, dim } = colors;
 
@@ -328,12 +330,30 @@ export async function runInit(
 	const agentCtx =
 		opts.agent === true || isAgentMode() || detectAgentCaller() !== null;
 	if (!opts.apiKey && !opts.email && agentCtx) {
+		const emitReuseEnvelope = (source: "env" | "config") => {
+			if (isAgentMode()) {
+				formatJsonEnvelope({
+					command: "init",
+					data: {
+						api_key_saved: false,
+						api_key_source: source,
+						agent_mode: false,
+						message:
+							"Existing Mem0 API key found and reused. No Agent Mode key was created.",
+					},
+				});
+			} else {
+				printSuccess(
+					source === "env"
+						? "Existing MEM0_API_KEY is valid; reusing it. No new Agent Mode key was minted."
+						: "Existing API key in config is valid; reusing it. No new Agent Mode key was minted.",
+				);
+			}
+		};
 		// Rule 1: env MEM0_API_KEY valid → reuse, no new key.
 		const envKey = (process.env.MEM0_API_KEY || "").trim();
 		if (envKey && (await pingKey(envKey, baseUrl))) {
-			printSuccess(
-				"Existing MEM0_API_KEY is valid; reusing it. No new Agent Mode key was minted.",
-			);
+			emitReuseEnvelope("env");
 			fireInit("existing_key");
 			return;
 		}
@@ -342,9 +362,7 @@ export async function runInit(
 			savedConfig.platform.apiKey &&
 			(await pingKey(savedConfig.platform.apiKey, baseUrl))
 		) {
-			printSuccess(
-				"Existing API key in config is valid; reusing it. No new Agent Mode key was minted.",
-			);
+			emitReuseEnvelope("config");
 			fireInit("existing_key");
 			return;
 		}

cli/node/src/index.ts

@@ -211,11 +211,19 @@ program
 		"--source <channel>",
 		"Channel attribution for signup (e.g. github, hn, ph).",
 	)
+	// Accept `--json` at the init level too so the PRD-documented form
+	// `mem0 init --agent --json` works without requiring users to move it
+	// before the subcommand. Effect is identical to the global `--json`:
+	// flip agent-mode output state.
+	.option("--json", "Output as JSON (alias for global `--json`).", false)
 	.addHelpText(
 		"after",
 		"\nExamples:\n  $ mem0 init\n  $ mem0 init --api-key m0-xxx --user-id alice\n  $ mem0 init --email you@example.com\n  $ mem0 init --email you@example.com --code 123456\n  $ mem0 init --agent             # Bootstrap an Agent Mode account (unattended)\n  $ mem0 init --email you@example.com  # Claims an existing Agent Mode key when one is present",
 	)
 	.action(async (opts) => {
+		// `--json` at init level mirrors the global flag — flip agent_mode
+		// state so downstream formatters use JSON envelopes.
+		if (opts.json) setAgentMode(true);
 		const { runInit } = await import("./commands/init.js");
 		await runInit({
 			apiKey: opts.apiKey,