feat: register all four worktree tools with placeholder implementations

Register git_worktree_list, git_worktree_create, git_worktree_switch,
and git_worktree_remove with correct TypeBox schemas. Each tool returns
'Not implemented yet.' as a placeholder response.

.pi/extensions/pi-worktree.ts

@@ -0,0 +1,70 @@
+import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
+import { Type } from "typebox";
+
+export default function (pi: ExtensionAPI) {
+  pi.registerTool({
+    name: "git_worktree_list",
+    label: "Git Worktree List",
+    description: "List all linked worktrees in the current repo with branch, path, and status.",
+    parameters: Type.Object({}),
+    async execute() {
+      return {
+        content: [{ type: "text", text: "Not implemented yet." }],
+      };
+    },
+  });
+
+  pi.registerTool({
+    name: "git_worktree_create",
+    label: "Git Worktree Create",
+    description: "Create a linked worktree for a branch in the current repo.",
+    parameters: Type.Object({
+      path: Type.String({ description: "Path for the new worktree" }),
+      branch: Type.Optional(
+        Type.String({
+          description: "Branch name (new or existing). Defaults to 'main'.",
+        })
+      ),
+      create_branch: Type.Optional(
+        Type.Boolean({
+          description: "Create a new branch? Defaults to true if branch doesn't exist.",
+        })
+      ),
+    }),
+    async execute() {
+      return {
+        content: [{ type: "text", text: "Not implemented yet." }],
+      };
+    },
+  });
+
+  pi.registerTool({
+    name: "git_worktree_switch",
+    label: "Git Worktree Switch",
+    description: "Switch pi's working directory to a linked worktree.",
+    parameters: Type.Object({
+      target: Type.String({
+        description: "Path or branch name of the target worktree",
+      }),
+    }),
+    async execute() {
+      return {
+        content: [{ type: "text", text: "Not implemented yet." }],
+      };
+    },
+  });
+
+  pi.registerTool({
+    name: "git_worktree_remove",
+    label: "Git Worktree Remove",
+    description: "Remove a linked worktree.",
+    parameters: Type.Object({
+      path: Type.String({ description: "Path of the worktree to remove" }),
+    }),
+    async execute() {
+      return {
+        content: [{ type: "text", text: "Not implemented yet." }],
+      };
+    },
+  });
+}