docs: 开发规范/教程/demo 全套 + 修复插件运行期自动加载缺口

- demo 插件 plugins/demo.hello-world：演示短代码过滤器、comment.created 钩子、路由+命名空间视图、迁移+模型、后台设置页、filament.post_form 表单注入
- demo 主题 themes/demo：最小可运行主题（覆盖 partials + 绿色系样式）
- 教程 docs/tutorial-plugin.md / docs/tutorial-theme.md（10 分钟上手）；规范 docs/development.md（代码/插件/主题/队列/测试/提交）
- README 增加开发者文档导航与上手示例
- 修复关键缺口：插件类原来靠 composer.json 硬编码加载，第三方 ZIP 安装的插件无法加载；现改为启动时扫描 plugins/*/src 运行期注册 PSR-4（register 阶段，保证 Filament 面板解析插件页面前就绪），composer.json 移除硬编码
- 测试：demo 插件 3 个用例 + demo 主题渲染；全量 89 通过

tests/Feature/DemoPluginTest.php

@@ -0,0 +1,70 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature;
+
+use App\Blog\Support\PluginManager;
+use App\Models\Comment;
+use App\Models\Post;
+use App\Models\Setting;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Plugins\Demo\HelloWorld\ServiceProvider;
+use Tests\TestCase;
+
+class DemoPluginTest extends TestCase
+{
+    use RefreshDatabase;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        // demo 插件默认未启用（不在 config/plugins.php enabled），测试中手动 boot 验证其代码
+        ServiceProvider::bootPlugin(app(PluginManager::class));
+    }
+
+    private function makePost(): Post
+    {
+        return Post::create([
+            'title' => '演示',
+            'slug' => 'demo-'.uniqid(),
+            'content' => '内容',
+            'content_format' => 'markdown',
+            'status' => 'published',
+            'published_at' => now(),
+        ]);
+    }
+
+    public function test_hello_shortcode_filter_renders_greeting(): void
+    {
+        Setting::set('hello_world_greeting', '嗨');
+
+        $html = app(PluginManager::class)->applyFilters('post.rendered', '正文 [hello 世界]', $this->makePost());
+
+        $this->assertStringContainsString('嗨，世界！', $html);
+        $this->assertStringNotContainsString('[hello', $html);
+    }
+
+    public function test_comment_created_action_writes_log(): void
+    {
+        $post = $this->makePost();
+        $comment = Comment::create([
+            'post_id' => $post->id,
+            'author_name' => '游客',
+            'content' => '演示评论内容',
+            'status' => 'pending',
+        ]);
+
+        app(PluginManager::class)->doAction('comment.created', $comment);
+
+        $this->assertDatabaseHas('hello_world_logs', ['comment_id' => $comment->id]);
+    }
+
+    public function test_hello_world_route_renders_plugin_view(): void
+    {
+        $this->get('/hello-world')
+            ->assertOk()
+            ->assertSee('Hello World 演示页');
+    }
+}