fix: verify prompt_versions migration exists, add model docblock

Investigation confirms migration exists at
Migrations/0001_01_01_000004_create_prompt_tables.php with all
required columns, foreign keys, and indexes:

- prompt_id FK (cascadeOnDelete)
- version (unsignedInteger)
- system_prompt, user_template (text, nullable)
- variables (json, nullable)
- created_by FK to users (nullOnDelete)
- composite index on [prompt_id, version]

Feature tests at tests/Feature/PromptVersionTest.php cover
creation, relationships, restore, and version history.

Adds @property docblock to PromptVersion model for IDE support,
consistent with AgentWorkspaceState.

Closes #22

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

Models/PromptVersion.php

@@ -8,6 +8,22 @@ use Core\Tenant\Models\User;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
+/**
+ * Prompt version snapshot for history and rollback.
+ *
+ * Captures the state of a prompt at a point in time, enabling
+ * version history and rollback via the restore() method.
+ *
+ * @property int $id
+ * @property int $prompt_id
+ * @property int $version
+ * @property string|null $system_prompt
+ * @property string|null $user_template
+ * @property array|null $variables
+ * @property int|null $created_by
+ * @property \Carbon\Carbon|null $created_at
+ * @property \Carbon\Carbon|null $updated_at
+ */
 class PromptVersion extends Model
 {
     protected $fillable = [