feat: initial release of relaticle/comments

Filament comments package with:
- Polymorphic commenting on any Eloquent model
- Threaded replies with configurable depth
- @mentions with autocomplete and user search
- Emoji reactions with toggle and who-reacted tooltips
- File attachments via Livewire uploads
- Reply and mention notifications via Filament notification system
- Thread subscriptions for notification control
- Real-time broadcasting (opt-in Echo) with polling fallback
- Dark mode support
- CommentsAction, CommentsTableAction, CommentsEntry for Filament integration
- 204 tests, 421 assertions

database/migrations/create_comment_subscriptions_table.php.stub

@@ -0,0 +1,20 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('comment_subscriptions', function (Blueprint $table) {
+            $table->id();
+            $table->morphs('commentable');
+            $table->morphs('user');
+            $table->timestamp('created_at')->nullable();
+
+            $table->unique(['commentable_type', 'commentable_id', 'user_type', 'user_id'], 'comment_subscriptions_unique');
+        });
+    }
+};