feat(todo): add an edit action, so correcting one item needs no rebuild

The vendored upstream example exposes list/add/toggle/clear and no way to
change an item's text. On a long-lived list that leaves two bad options:
add a "patch" item that describes a DIFFERENT item, or clear and re-add
everything. Both were hit for real on 2026-08-17 while tracking a 17-item
fleet plan -- the list ended up with #18 correcting #17, which reads as
nonsense in a sidebar that truncates each item to a few words.

edit takes id + text, replaces the text, and KEEPS the id and the done
status. Id stability is the point: ids are the only handle the palace
snapshot of a plan can refer to, so a correction must not renumber
anything. nextId is untouched.

Error branches match the existing style: distinct messages for a missing
id, a missing text, and an unknown id, each returning the full todos array
in details so state stays consistent.

No pi-atelier change needed. Its tool_result hook only checks that
details.todos is a well-shaped array and ignores the action string, so the
new action flows through its normalizer and sidebar untouched.

Note renderResult's switch has no default case, so an action without a
render arm returns undefined; edit adds one.

The file is a verbatim vendored copy of upstream's example, so divergence
has a cost: the header now carries a numbered LOCAL DELTAS list (the
earlier ctx.mode -> ctx.hasUI API fix, and this) to keep reconciling a
future upstream version mechanical. Worth offering upstream as a PR -- the
example is arguably incomplete without it.

Verified live in-container by pointing ~/.pi/agent/extensions/todo.ts at
this copy for one session: edit on an unknown id and with no text both
error as intended, and editing a completed item preserved both its id and
its done state. Symlink since reverted to the image copy.

extensions/todo.ts

@@ -8,6 +8,15 @@
  * State is stored in tool result details (not external files), which allows
  * proper branching - when you branch, the todo state is automatically
  * correct for that point in history.
+ *
+ * LOCAL DELTAS vs upstream examples/extensions/todo.ts (keep this list current,
+ * so reconciling a future upstream version stays mechanical):
+ *   1. `ctx.mode !== "tui"` -> `!ctx.hasUI` in the /todos command (API change).
+ *   2. `edit` action (id + text) - rewrite an item's text in place, keeping its
+ *      id and done status. Added 2026-08-17: without it, correcting one item in
+ *      a long-lived list means either a "patch" item that describes a DIFFERENT
+ *      item, or clear + re-add of everything. Both were hit in practice on
+ *      2026-08-17 while tracking a 17-item fleet plan.
  */
 
 import { StringEnum } from "@earendil-works/pi-ai";
@@ -22,16 +31,16 @@ interface Todo {
 }
 
 interface TodoDetails {
-	action: "list" | "add" | "toggle" | "clear";
+	action: "list" | "add" | "toggle" | "edit" | "clear";
 	todos: Todo[];
 	nextId: number;
 	error?: string;
 }
 
 const TodoParams = Type.Object({
-	action: StringEnum(["list", "add", "toggle", "clear"] as const),
-	text: Type.Optional(Type.String({ description: "Todo text (for add)" })),
-	id: Type.Optional(Type.Number({ description: "Todo ID (for toggle)" })),
+	action: StringEnum(["list", "add", "toggle", "edit", "clear"] as const),
+	text: Type.Optional(Type.String({ description: "Todo text (for add, or the replacement text for edit)" })),
+	id: Type.Optional(Type.Number({ description: "Todo ID (for toggle and edit)" })),
 });
 
 /**
@@ -136,7 +145,8 @@ export default function (pi: ExtensionAPI) {
 	pi.registerTool({
 		name: "todo",
 		label: "Todo",
-		description: "Manage a todo list. Actions: list, add (text), toggle (id), clear",
+		description:
+			"Manage a todo list. Actions: list, add (text), toggle (id), edit (id + text, replaces the text and keeps the done status), clear",
 		parameters: TodoParams,
 
 		async execute(_toolCallId, params, _signal, _onUpdate, _ctx) {
@@ -195,6 +205,38 @@ export default function (pi: ExtensionAPI) {
 					};
 				}
 
+				case "edit": {
+					if (params.id === undefined || !params.text) {
+						const missing = params.id === undefined ? "id" : "text";
+						return {
+							content: [{ type: "text", text: `Error: ${missing} required for edit` }],
+							details: {
+								action: "edit",
+								todos: [...todos],
+								nextId,
+								error: `${missing} required`,
+							} as TodoDetails,
+						};
+					}
+					const target = todos.find((t) => t.id === params.id);
+					if (!target) {
+						return {
+							content: [{ type: "text", text: `Todo #${params.id} not found` }],
+							details: {
+								action: "edit",
+								todos: [...todos],
+								nextId,
+								error: `#${params.id} not found`,
+							} as TodoDetails,
+						};
+					}
+					target.text = params.text;
+					return {
+						content: [{ type: "text", text: `Todo #${target.id} updated: ${target.text}` }],
+						details: { action: "edit", todos: [...todos], nextId } as TodoDetails,
+					};
+				}
+
 				case "clear": {
 					const count = todos.length;
 					todos = [];
@@ -274,6 +316,12 @@ export default function (pi: ExtensionAPI) {
 					return new Text(theme.fg("success", "✓ ") + theme.fg("muted", msg), 0, 0);
 				}
 
+				case "edit": {
+					const text = result.content[0];
+					const msg = text?.type === "text" ? text.text : "";
+					return new Text(theme.fg("success", "✎ ") + theme.fg("muted", msg), 0, 0);
+				}
+
 				case "clear":
 					return new Text(theme.fg("success", "✓ ") + theme.fg("muted", "Cleared all todos"), 0, 0);
 			}