feat(setup): label headless URL fallback with "Get started:"

When a card's auto-open is gated on `confirmThenOpen`, the URL also
appears inside the surrounding `note(...)` as a copy-paste fallback —
rendered dim because on a GUI device the auto-open is doing the
heavy lifting and the printed URL is just an incidental backup.

On headless devices the auto-open doesn't run (per #2145), so the
URL inside the note is the user's *only* path forward. A dim URL
reads as "incidental reference" exactly when it should be reading
as "this is the action."

Adds `formatNoteLink(url)` to setup/lib/browser.ts:
  - GUI device → `k.dim(url)` (unchanged from today)
  - Headless device → `Get started: <url>` at full strength

Replaces five call sites (Discord ×3, Slack ×1, Telegram ×1).
Single helper, atomic switch via the same `isHeadless()` plumbing
introduced in #2145, so the headless behavior across all five
flows stays in sync.

setup/lib/browser.ts

@@ -19,6 +19,7 @@
 import { spawn } from 'child_process';
 
 import * as p from '@clack/prompts';
+import k from 'kleur';
 
 import { isHeadless } from '../platform.js';
 import { ensureAnswer } from './runner.js';
@@ -38,6 +39,21 @@ export function openUrl(url: string): void {
   }
 }
 
+/**
+ * Format a URL for display inside a setup `note(...)` card. On
+ * GUI devices the URL renders dim — it's a fallback in case the
+ * auto-open misses, and `confirmThenOpen` is doing the heavy
+ * lifting of getting the user there. On headless devices the
+ * URL becomes the user's only path forward, so we surface it
+ * with a "Get started:" label and full-strength text — copy-
+ * pasting onto another device is the actual action, not an
+ * incidental reference.
+ */
+export function formatNoteLink(url: string): string {
+  if (isHeadless()) return `Get started: ${url}`;
+  return k.dim(url);
+}
+
 /**
  * Gate a browser-open on a confirm so the user is ready for their browser
  * to take focus. Proceeds on cancel as well — the user can always copy the