fix(builder): item 15 — default header nav no longer links nonexistent pages

DEFAULT_HEADER_STATE's Navbar linked Home/About/Services/Contact, but a
brand new site only has a Home page — About/Services/Contact were dead
links on first click. Simplified the default to Home + an inert "Get
Started" CTA (href: '#') rather than seeding placeholder pages nobody
asked for. Updated default-header.test.ts's HTML-export assertions to
match the new default (and assert the removed links are gone).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/state/default-header.test.ts

@@ -23,8 +23,14 @@ describe('DEFAULT_HEADER_STATE seed', () => {
     const { html } = exportBodyHtml(DEFAULT_HEADER_STATE);
     expect(html).toContain('<nav');
     expect(html).toContain('MySite');
-    for (const link of ['Home', 'About', 'Services', 'Contact']) {
+    // Item 15: the default nav only links pages/anchors that actually exist
+    // on a brand new site (just "Home") plus an inert CTA -- no dead links
+    // to About/Services/Contact pages that were never seeded.
+    for (const link of ['Home', 'Get Started']) {
       expect(html).toContain(link);
     }
+    expect(html).not.toContain('About');
+    expect(html).not.toContain('Services');
+    expect(html).not.toContain('Contact');
   });
 });