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>

app/Http/Resources/SiteResource.php

@@ -17,6 +17,7 @@ class SiteResource extends JsonResource
             'cms_type' => $this->cms_type,
             'wp_username' => $this->wp_username,
             'wp_app_password' => $this->wp_app_password,
+            'api_instructions' => $this->api_instructions,
             'created_at' => $this->created_at,
             'updated_at' => $this->updated_at,
         ];