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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pi-worktree/package.json

@@ -0,0 +1,46 @@
+{
+	"name": "pi-worktree",
+	"version": "0.1.0",
+	"description": "Slash-command git worktree manager for Pi: one-click split and merge.",
+	"private": true,
+	"type": "module",
+	"license": "MIT",
+	"main": "./index.ts",
+	"exports": {
+		".": "./index.ts"
+	},
+	"files": [
+		"index.ts",
+		"src",
+		"README.md",
+		"LICENSE"
+	],
+	"pi": {
+		"extensions": [
+			"./index.ts"
+		]
+	},
+	"scripts": {
+		"test": "node --test test/*.test.ts",
+		"check": "node --experimental-strip-types --check index.ts && node --test test/*.test.ts"
+	},
+	"peerDependencies": {
+		"@earendil-works/pi-coding-agent": "*",
+		"@earendil-works/pi-tui": "*",
+		"typebox": "*"
+	},
+	"peerDependenciesMeta": {
+		"@earendil-works/pi-coding-agent": {
+			"optional": true
+		},
+		"@earendil-works/pi-tui": {
+			"optional": true
+		},
+		"typebox": {
+			"optional": true
+		}
+	},
+	"engines": {
+		"node": ">=22.19.0"
+	}
+}