Add free-text api_instructions field to Sites

For custom sites we'll post to via their own API — there's no wired-in
publisher for these, so this is where the site owner documents how to
actually post there (auth, endpoints, payload shape). Shown in the UI
for non-WordPress sites, exposed via the JSON API, and pointed to from
AI_API.md's "any other network/CMS" fallback section so an agent checks
it before improvising.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/Feature/Api/SitesApiTest.php

@@ -46,3 +46,15 @@ test('it includes WordPress fields in the site response', function () {
         ->assertJsonPath('data.wp_username', 'admin')
         ->assertJsonPath('data.wp_app_password', 'abcd 1234 efgh 5678');
 });
+
+test('it includes api_instructions in the site response', function () {
+    $site = Site::factory()->create([
+        'cms_type' => 'custom',
+        'api_instructions' => 'POST to /api/publish with a bearer token from .env',
+    ]);
+
+    $response = $this->getJson("/api/sites/{$site->id}");
+
+    $response->assertOk()
+        ->assertJsonPath('data.api_instructions', 'POST to /api/publish with a bearer token from .env');
+});