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

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

pi-worktree/test/commands.test.ts

@@ -0,0 +1,64 @@
+import assert from "node:assert/strict";
+import { mkdtemp } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import test from "node:test";
+
+import { argumentCompletions } from "../index.ts";
+import { addEntry, updateState, type WorktreeEntry } from "../src/state.ts";
+
+function entry(overrides: Partial<WorktreeEntry> = {}): WorktreeEntry {
+	return {
+		slug: "demo",
+		path: "/nonexistent/demo",
+		branch: "wt/demo",
+		sourcePath: "/nonexistent/source",
+		sourceBranch: "main",
+		sourceSha: "0".repeat(40),
+		createdAt: new Date().toISOString(),
+		...overrides,
+	};
+}
+
+test("first-token completion returns subcommand names", async () => {
+	const items = await argumentCompletions("mer");
+	assert.ok(items?.some((item) => item.value === "merge"));
+});
+
+test("merge flag completions expand to the full argument line", async () => {
+	const items = await argumentCompletions("merge --c");
+	assert.deepEqual(
+		items?.map((item) => ({ value: item.value, label: item.label })),
+		[{ value: "merge --continue", label: "--continue" }],
+	);
+});
+
+test("bare -- after merge offers all four flags as full lines", async () => {
+	const items = await argumentCompletions("merge --");
+	assert.equal(items?.length, 4);
+	for (const item of items ?? []) {
+		assert.ok(item.value.startsWith("merge --"), item.value);
+	}
+});
+
+test("slug completions keep the subcommand in the replacement value", async (t) => {
+	const agentDir = await mkdtemp(join(tmpdir(), "pi-worktree-agent-"));
+	const previous = process.env.PI_CODING_AGENT_DIR;
+	process.env.PI_CODING_AGENT_DIR = agentDir;
+	t.after(() => {
+		if (previous === undefined) delete process.env.PI_CODING_AGENT_DIR;
+		else process.env.PI_CODING_AGENT_DIR = previous;
+	});
+	await updateState((s) => addEntry(s, entry()));
+
+	const switchItems = await argumentCompletions("switch ");
+	const values = switchItems?.map((item) => item.value) ?? [];
+	assert.ok(values.includes("switch main"), JSON.stringify(values));
+	assert.ok(values.includes("switch demo"), JSON.stringify(values));
+
+	const removeItems = await argumentCompletions("remove d");
+	assert.deepEqual(
+		removeItems?.map((item) => ({ value: item.value, label: item.label })),
+		[{ value: "remove demo", label: "demo" }],
+	);
+});