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/SiteModelTest.php

@@ -50,3 +50,20 @@ test('a site can be created as WordPress with credentials', function () {
         ->wp_username->toBe('admin')
         ->wp_app_password->toBe('abcd 1234 efgh 5678');
 });
+
+test('a custom site can store free-text api_instructions', function () {
+    $site = Site::create([
+        'name' => 'Custom Site',
+        'url' => 'https://custom.example.com',
+        'cms_type' => 'custom',
+        'api_instructions' => 'POST to /api/publish with a bearer token from .env',
+    ]);
+
+    expect($site->fresh()->api_instructions)->toBe('POST to /api/publish with a bearer token from .env');
+});
+
+test('api_instructions defaults to null', function () {
+    $site = Site::create(['name' => 'My Blog', 'url' => 'https://example.com']);
+
+    expect($site->fresh()->api_instructions)->toBeNull();
+});