feat(pi-worktree): 新增斜杠命令 git worktree 管理扩展

提供 /wt（别名 /worktree）一键拆分/合并：
- /wt new 建 worktree 后经 SessionManager.forkFrom + switchSession
  迁移会话 cwd，历史上下文保留；合并/切换同理
- /wt merge 先确认再 rebase 到来源分支、默认 --ff-only（可选
  --squash），来源改动用 stash 兜底，冲突可中止或保留后用
  /wt merge --continue / --abort 处理，永不 push
- 来源 checkout 的 .pi/worktree.json 声明环境复刻（copy/link/env/
  postCreate），确认框先展示计划再执行；postCreate 仅在项目已信任
  时以 argv 形式 execFile，失败可 /wt reopen 增量重跑
- 登记文件 <agentDir>/pi-worktree/state.json 原子写入并加锁
  串行化；会话切换后向模型注入 cwd 变更提示；未被 .gitignore
  覆盖的软链/环境文件写入 .git/info/exclude 防止误提交
- 仅注册用户显式触发的斜杠命令，不暴露模型可调用工具

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tests/extension-load.test.ts

@@ -72,6 +72,12 @@ test("all package extensions load together without global registration conflicts
 	assert.ok(packageJson.pi.extensions.includes("./pi-ask-user/index.ts"), "the bundle must load pi-ask-user");
 	assert.equal(packageJson.dependencies["pi-ask-user"], "file:./pi-ask-user");
 	assert.ok(packageJson.files.includes("pi-ask-user"), "the packed bundle must include pi-ask-user source");
+	assert.ok(
+		packageJson.pi.extensions.includes("./pi-worktree/index.ts"),
+		"the bundle must load the locally maintained worktree commands",
+	);
+	assert.equal(packageJson.dependencies["pi-worktree"], "file:./pi-worktree");
+	assert.ok(packageJson.files.includes("pi-worktree"), "the packed bundle must include pi-worktree source");
 	const chromeExtension = "./pi-chrome/extensions/chrome-profile-bridge/index.ts";
 	assert.equal(packageJson.dependencies["pi-chrome"], "file:./pi-chrome", "the browser bridge must use the locally maintained source");
 	assert.ok(packageJson.files.includes("pi-chrome"), "the packed bundle must include the locally maintained pi-chrome source");