feat: add SocialMediaAssistant agent with Conversational interface

The agent implements Laravel AI SDK's Agent + Conversational contracts:
- instructions() renders the existing Blade system prompt with workspace
  brand context (name, description, website, tone, voice notes, locale)
- messages() reads directly from the AiMessage model scoped to the post,
  so our existing conversation persistence stays the single source of
  truth — no duplicate SDK-managed storage.
- Assistant messages with attachments are enriched inline with a summary
  like [This assistant message attached: 2 image] so the model tracks
  progress through carousel-style multi-image generations.
- provider() maps the trypost.ai.text_provider config to Lab::Gemini or
  Lab::OpenAI, preserving the existing provider-switching behavior.

app/Ai/Agents/SocialMediaAssistant.php

@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Ai\Agents;
+
+use App\Models\AiMessage;
+use App\Models\Post;
+use App\Models\Workspace;
+use Laravel\Ai\Contracts\Agent;
+use Laravel\Ai\Contracts\Conversational;
+use Laravel\Ai\Enums\Lab;
+use Laravel\Ai\Messages\Message;
+use Laravel\Ai\Promptable;
+
+class SocialMediaAssistant implements Agent, Conversational
+{
+    use Promptable;
+
+    public function __construct(
+        public Workspace $workspace,
+        public ?Post $post = null,
+        public ?string $userId = null,
+    ) {}
+
+    public function instructions(): string
+    {
+        return view('prompts.assistant.system', [
+            'brand_name' => $this->workspace->name ?? '',
+            'brand_description' => $this->workspace->brand_description ?? '',
+            'brand_website' => $this->workspace->brand_website ?? '',
+            'tone' => $this->workspace->brand_tone ?? 'professional',
+            'voice_notes' => $this->workspace->brand_voice_notes ?? '',
+            'locale' => app()->getLocale(),
+        ])->render();
+    }
+
+    /**
+     * @return iterable<Message>
+     */
+    public function messages(): iterable
+    {
+        if (! $this->post) {
+            return [];
+        }
+
+        return AiMessage::query()
+            ->where('post_id', $this->post->id)
+            ->whereIn('role', ['user', 'assistant'])
+            ->oldest()
+            ->limit(20)
+            ->get()
+            ->map(fn (AiMessage $m) => new Message($m->role, $this->enrichContent($m)))
+            ->all();
+    }
+
+    public function provider(): Lab
+    {
+        return match (config('trypost.ai.text_provider')) {
+            'openai' => Lab::OpenAI,
+            default => Lab::Gemini,
+        };
+    }
+
+    private function enrichContent(AiMessage $m): string
+    {
+        $content = $m->content;
+
+        if ($m->role === 'assistant' && ! empty($m->attachments)) {
+            $counts = collect($m->attachments)
+                ->groupBy('type')
+                ->map(fn ($group, $type) => count($group)." {$type}")
+                ->implode(', ');
+
+            $content .= "\n\n[This assistant message attached: {$counts}]";
+        }
+
+        return $content;
+    }
+}

tests/Feature/Ai/SocialMediaAssistantTest.php

@@ -0,0 +1,108 @@
+<?php
+
+declare(strict_types=1);
+
+use App\Ai\Agents\SocialMediaAssistant;
+use App\Enums\User\Setup;
+use App\Enums\UserWorkspace\Role;
+use App\Models\AiMessage;
+use App\Models\Post;
+use App\Models\User;
+use App\Models\Workspace;
+use Laravel\Ai\Enums\Lab;
+use Laravel\Ai\Messages\Message;
+
+beforeEach(function () {
+    $this->user = User::factory()->create(['setup' => Setup::Completed]);
+    $this->workspace = Workspace::factory()->create([
+        'user_id' => $this->user->id,
+        'name' => 'Paulo Coffee',
+        'brand_tone' => 'friendly',
+    ]);
+    $this->workspace->members()->attach($this->user->id, ['role' => Role::Member->value]);
+    $this->user->update(['current_workspace_id' => $this->workspace->id]);
+
+    $this->post = Post::factory()->create([
+        'workspace_id' => $this->workspace->id,
+        'user_id' => $this->user->id,
+    ]);
+});
+
+test('instructions include brand context from workspace', function () {
+    $agent = new SocialMediaAssistant($this->workspace, $this->post);
+
+    $instructions = $agent->instructions();
+
+    expect($instructions)
+        ->toContain('Paulo Coffee')
+        ->toContain('friendly');
+});
+
+test('messages returns AiMessage rows as SDK messages', function () {
+    AiMessage::factory()->create([
+        'post_id' => $this->post->id,
+        'role' => 'user',
+        'content' => 'Write a caption',
+    ]);
+    AiMessage::factory()->create([
+        'post_id' => $this->post->id,
+        'role' => 'assistant',
+        'content' => 'Here it is!',
+    ]);
+
+    $agent = new SocialMediaAssistant($this->workspace, $this->post);
+
+    $messages = collect($agent->messages())->all();
+
+    expect($messages)->toHaveCount(2);
+    expect($messages[0])->toBeInstanceOf(Message::class);
+    expect($messages[0]->role->value)->toBe('user');
+    expect($messages[0]->content)->toBe('Write a caption');
+    expect($messages[1]->role->value)->toBe('assistant');
+    expect($messages[1]->content)->toBe('Here it is!');
+});
+
+test('messages is empty when no post is provided', function () {
+    $agent = new SocialMediaAssistant($this->workspace);
+
+    $messages = collect($agent->messages())->all();
+
+    expect($messages)->toBeEmpty();
+});
+
+test('messages is empty when post has no AiMessages', function () {
+    $agent = new SocialMediaAssistant($this->workspace, $this->post);
+
+    $messages = collect($agent->messages())->all();
+
+    expect($messages)->toBeEmpty();
+});
+
+test('messages enriches assistant content with attachment summary', function () {
+    AiMessage::factory()->create([
+        'post_id' => $this->post->id,
+        'role' => 'assistant',
+        'content' => 'Here is your image',
+        'attachments' => [
+            ['id' => 'a', 'type' => 'image'],
+            ['id' => 'b', 'type' => 'image'],
+        ],
+    ]);
+
+    $agent = new SocialMediaAssistant($this->workspace, $this->post);
+
+    $messages = collect($agent->messages())->all();
+
+    expect($messages[0]->content)->toContain('Here is your image');
+    expect($messages[0]->content)->toContain('[This assistant message attached: 2 image]');
+});
+
+test('provider honors trypost.ai.text_provider config', function () {
+    config()->set('trypost.ai.text_provider', 'openai');
+
+    expect((new SocialMediaAssistant($this->workspace))->provider())->toBe(Lab::OpenAI);
+
+    config()->set('trypost.ai.text_provider', 'gemini');
+
+    expect((new SocialMediaAssistant($this->workspace))->provider())->toBe(Lab::Gemini);
+});