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

package-lock.json

@@ -25,6 +25,7 @@
         "pi-mcp-adapter": "2.26.0",
         "pi-ssh": "file:./pi-ssh",
         "pi-tool-search": "file:./pi-tool-search",
+        "pi-worktree": "file:./pi-worktree",
         "ssh2": "1.17.0",
         "typescript": "6.0.3",
         "typescript-language-server": "5.3.0",
@@ -7324,6 +7325,10 @@
       "resolved": "pi-tool-search",
       "link": true
     },
+    "node_modules/pi-worktree": {
+      "resolved": "pi-worktree",
+      "link": true
+    },
     "node_modules/picocolors": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
@@ -8941,6 +8946,29 @@
           "optional": true
         }
       }
+    },
+    "pi-worktree": {
+      "version": "0.1.0",
+      "license": "MIT",
+      "engines": {
+        "node": ">=22.19.0"
+      },
+      "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
+        }
+      }
     }
   }
 }