fix(models): add context array cast to AgentPlan

The context column (longText) was missing its array cast, causing
"Array to string conversion" errors when creating plans via MCP.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Models/AgentPlan.php

@@ -63,6 +63,7 @@ class AgentPlan extends Model
     ];
 
     protected $casts = [
+        'context' => 'array',
         'phases' => 'array',
         'metadata' => 'array',
     ];