docs: dedicate /platform/agent-signup page; rework introduction + nav

Per launch feedback (Deshraj, Mragank, Rudraj on 2026-05-16):

- NEW: docs/platform/agent-signup.mdx — dedicated tutorial page. Owns
  the full Agent Mode story: 4-command demo, --agent-caller, mem0
  identify fallback, claim flow, rate limits, what's-next links.

- docs/docs.json: insert platform/agent-signup at position 2 in the
  Platform > Getting Started group (directly under platform/overview).

- docs/introduction.mdx: the homepage card formerly labeled "CLI" and
  linking to /platform/cli now points to /platform/agent-signup with
  the headline "Sign up as an agent" — the first big call-to-action.

- docs/platform/cli.mdx: stripped the Agent Mode promo block, the
  Plugin Sync section, the agent-related rows from the mem0 init flag
  table, and the entire mem0 identify subsection. Added a one-line
  wayfinding Tip + Note pointing readers to /platform/agent-signup.
  CLI page is now a clean reference for human dashboard users.

- docs/platform/quickstart.mdx: reverted the duplicate 4-step block
  added in the earlier commit. Replaced with a one-line Note pointing
  to /platform/agent-signup.

- README.md: closing pointer added to the existing Quickstart subtitle
  so the GitHub landing page lands readers in the docs.

docs/introduction.mdx

@@ -157,27 +157,27 @@ mode: "custom"
     </a>
 
     <a
-      href="/platform/cli"
+      href="/platform/agent-signup"
       className="group flex h-full flex-col overflow-hidden rounded-2xl border border-gray-200 dark:border-zinc-800/40 bg-white dark:bg-zinc-900/40 transition hover:border-primary/60 hover:bg-gray-50 dark:hover:bg-zinc-900"
     >
       <img
         className="block dark:hidden aspect-[2/1] w-full object-cover"
         src="/images/docs thumbnails/light/CLI.png"
-        alt="CLI thumbnail"
+        alt="Sign up as an agent thumbnail"
         style={{pointerEvents: "none"}}
       />
       <img
         className="hidden dark:block aspect-[2/1] w-full object-cover"
         src="/images/docs thumbnails/dark/CLI.png"
-        alt="CLI thumbnail"
+        alt="Sign up as an agent thumbnail"
         style={{pointerEvents: "none"}}
       />
       <div className="flex flex-1 flex-col gap-2 px-4 pb-4 pt-3 text-left">
         <h3 className="text-base font-semibold text-gray-900 dark:text-zinc-100 group-hover:text-primary">
-          CLI
+          Sign up as an agent
         </h3>
         <p className="text-sm text-gray-600 dark:text-zinc-400">
-          Manage memories directly from your terminal. Built for developers and AI agents.
+          For AI agents: mint a Mem0 API key in under five seconds — no email, no dashboard. Four commands to your first memory.
         </p>
       </div>
     </a>