feat: 添加 Commands 系统支持斜杠命令

实现类似 OpenCode 的 Commands 功能：
- 支持 Markdown 格式定义命令，带 YAML frontmatter
- 变量替换：$ARGUMENTS, $1/$2, @filepath, !`shell`
- 三级加载：builtin < user < project
- 7 个内置命令：init, review, test, fix, explain, commit, help
- 集成终端 UI 支持 /commands 列表和命令执行
- 完整单元测试覆盖 (46 tests)

src/commands/registry.ts

@@ -0,0 +1,199 @@
+/**
+ * Command 注册表
+ *
+ * 管理所有可用的 Commands，支持：
+ * - 注册/注销 Commands
+ * - 按名称查询
+ * - 搜索 Commands
+ */
+
+import type { Command, CommandSearchResult } from './types.js';
+import { commandLoader } from './loader.js';
+import { builtinCommands } from './builtin/index.js';
+
+/**
+ * Command 注册表
+ */
+export class CommandRegistry {
+  private commands = new Map<string, Command>();
+  private initialized = false;
+
+  /**
+   * 初始化注册表
+   */
+  async initialize(workdir: string = process.cwd()): Promise<void> {
+    if (this.initialized) {
+      return;
+    }
+
+    // 1. 注册内置 Commands
+    for (const command of builtinCommands) {
+      this.register(command);
+    }
+
+    // 2. 加载用户 Commands
+    const userDir = commandLoader.getUserCommandsDir();
+    const userCommands = await commandLoader.loadFromDirectory(userDir, 'user');
+    for (const command of userCommands) {
+      this.register(command);
+    }
+
+    // 3. 加载项目 Commands
+    const projectDir = commandLoader.getProjectCommandsDir(workdir);
+    const projectCommands = await commandLoader.loadFromDirectory(
+      projectDir,
+      'project'
+    );
+    for (const command of projectCommands) {
+      this.register(command);
+    }
+
+    this.initialized = true;
+  }
+
+  /**
+   * 注册 Command
+   */
+  register(command: Command): void {
+    // 项目 Commands 优先级最高，可以覆盖同名的内置/用户 Commands
+    const existing = this.commands.get(command.name);
+    if (existing) {
+      // 优先级: project > user > builtin
+      const priority = { project: 3, user: 2, builtin: 1 };
+      if (priority[command.source] < priority[existing.source]) {
+        return; // 不覆盖更高优先级的 Command
+      }
+    }
+    this.commands.set(command.name, command);
+  }
+
+  /**
+   * 注销 Command
+   */
+  unregister(name: string): boolean {
+    return this.commands.delete(name);
+  }
+
+  /**
+   * 获取 Command
+   */
+  get(name: string): Command | undefined {
+    return this.commands.get(name);
+  }
+
+  /**
+   * 检查 Command 是否存在
+   */
+  has(name: string): boolean {
+    return this.commands.has(name);
+  }
+
+  /**
+   * 获取所有 Commands
+   */
+  getAll(): Command[] {
+    return Array.from(this.commands.values());
+  }
+
+  /**
+   * 搜索 Commands
+   */
+  search(query: string, limit: number = 10): CommandSearchResult[] {
+    const queryLower = query.toLowerCase();
+    const results: CommandSearchResult[] = [];
+
+    for (const command of this.commands.values()) {
+      let score = 0;
+
+      // 精确名称匹配
+      if (command.name.toLowerCase() === queryLower) {
+        score = 100;
+      }
+      // 名称前缀匹配
+      else if (command.name.toLowerCase().startsWith(queryLower)) {
+        score = 80;
+      }
+      // 名称包含匹配
+      else if (command.name.toLowerCase().includes(queryLower)) {
+        score = 60;
+      }
+      // 描述匹配
+      else if (command.description?.toLowerCase().includes(queryLower)) {
+        score = 40;
+      }
+
+      if (score > 0) {
+        results.push({ command, score });
+      }
+    }
+
+    // 按分数降序排序
+    results.sort((a, b) => b.score - a.score);
+
+    return results.slice(0, limit);
+  }
+
+  /**
+   * 列出所有 Commands（用于帮助显示）
+   */
+  list(): Array<{ name: string; description?: string; source: string }> {
+    return this.getAll()
+      .map((cmd) => ({
+        name: cmd.name,
+        description: cmd.description,
+        source: cmd.source,
+      }))
+      .sort((a, b) => a.name.localeCompare(b.name));
+  }
+
+  /**
+   * 重新加载 Commands
+   */
+  async reload(workdir: string = process.cwd()): Promise<void> {
+    this.commands.clear();
+    this.initialized = false;
+    await this.initialize(workdir);
+  }
+
+  /**
+   * 获取统计信息
+   */
+  getStats(): {
+    total: number;
+    bySource: Record<string, number>;
+  } {
+    const commands = this.getAll();
+    const bySource: Record<string, number> = {};
+
+    for (const command of commands) {
+      bySource[command.source] = (bySource[command.source] || 0) + 1;
+    }
+
+    return {
+      total: commands.length,
+      bySource,
+    };
+  }
+}
+
+/**
+ * 全局 Command 注册表实例
+ */
+let commandRegistryInstance: CommandRegistry | null = null;
+
+/**
+ * 获取全局 Command 注册表
+ */
+export function getCommandRegistry(): CommandRegistry {
+  if (!commandRegistryInstance) {
+    commandRegistryInstance = new CommandRegistry();
+  }
+  return commandRegistryInstance;
+}
+
+/**
+ * 重置全局 Command 注册表（用于测试）
+ */
+export function resetCommandRegistry(): void {
+  commandRegistryInstance = null;
+}