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/channels/slack.ts

@@ -25,7 +25,7 @@ import * as p from '@clack/prompts';
 import k from 'kleur';
 
 import * as setupLog from '../logs.js';
-import { confirmThenOpen } from '../lib/browser.js';
+import { confirmThenOpen, formatNoteLink } from '../lib/browser.js';
 import { askOperatorRole } from '../lib/role-prompt.js';
 import { ensureAnswer, fail, runQuietChild } from '../lib/runner.js';
 import { accentGreen, fmtDuration, note, wrapForGutter } from '../lib/theme.js';
@@ -136,7 +136,7 @@ async function walkThroughAppCreation(): Promise<void> {
       '  4. Basic Information → copy the "Signing Secret"',
       '  5. Install to Workspace → copy the "Bot User OAuth Token" (xoxb-…)',
       '',
-      k.dim(SLACK_APPS_URL),
+      formatNoteLink(SLACK_APPS_URL),
     ].join('\n'),
     'Create a Slack app',
   );