Add todo.ts (verbatim copy of upstream examples/extensions/todo.ts)

Provides the agent with a 'todo' tool (list/add/toggle/clear) and
registers /todos for the user. Useful for externalising multi-step
plans during long arcs.

State persists in tool result details rather than an external file,
which means: pi --continue brings todos back with the session, and
/fork forks the todo state along with the branch.

Copied not symlinked because the upstream path lives under a
homebrew-versioned Cellar dir that rotates on every pi upgrade.
Refresh procedure documented in AGENTS.md.

README.md

@@ -193,6 +193,16 @@ A footer line shows pending changes (e.g. `pending: notify→off, foo→on`) so
 3. In a running pi session, `/reload` is enough; no restart needed
 4. (or, with `ext-toggle` installed: `/ext` to disable noisy ones at runtime)
 
+### `todo.ts`
+
+Gives the agent a `todo` tool (actions: `list` / `add` / `toggle` / `clear`) so it can externalize a multi-step plan and tick items off as it works. Also registers `/todos` so you can inspect the current list at any time.
+
+State lives in the session's tool result `details`, not an external file. So:
+- `pi --continue` / `--resume` brings the todos back with the conversation.
+- `/fork` forks the todo list along with the branch — each branch has its own state.
+
+This is a verbatim copy of the upstream `examples/extensions/todo.ts` shipped with `pi-coding-agent`. Refresh from upstream when desired (see `AGENTS.md`).
+
 Each extension is a TypeScript module loaded by [jiti](https://github.com/unjs/jiti) — no compilation step. See the [pi extensions docs](https://github.com/mariozechner/pi-coding-agent/blob/main/docs/extensions.md) and the [built-in examples](https://github.com/mariozechner/pi-coding-agent/tree/main/examples/extensions) for the API surface.
 
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