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/Livewire/SitesTest.php

@@ -121,3 +121,24 @@ test('switching a WordPress site to a non-WordPress cms_type clears its credenti
         ->wp_username->toBeNull()
         ->wp_app_password->toBeNull();
 });
+
+test('a custom site can be created with api_instructions', function () {
+    Livewire::test(Index::class)
+        ->set('name', 'Custom Site')
+        ->set('url', 'https://custom.example.com')
+        ->set('cmsType', 'custom')
+        ->set('apiInstructions', 'POST to /api/publish with a bearer token from .env')
+        ->call('save');
+
+    $site = Site::where('name', 'Custom Site')->first();
+
+    expect($site->api_instructions)->toBe('POST to /api/publish with a bearer token from .env');
+});
+
+test('editing a site loads its api_instructions', function () {
+    $site = Site::factory()->create(['cms_type' => 'custom', 'api_instructions' => 'Use the /publish endpoint']);
+
+    Livewire::test(Index::class)
+        ->call('edit', $site->id)
+        ->assertSet('apiInstructions', 'Use the /publish endpoint');
+});