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.

AGENTS.md

@@ -22,6 +22,7 @@ extensions/
   git-checkpoint.ts       # Git stash checkpoint per turn, restorable on /fork
   notify.ts               # Native terminal notification when agent finishes
   ext-toggle.ts           # /ext slash command — list & toggle extensions at runtime
+  todo.ts                 # `todo` tool for the agent + /todos for the user (copy of upstream example)
 install.sh                # Idempotent installer — symlinks extensions/ into ~/.pi/agent/extensions/
 package.json              # pi package manifest — enables `pi install /path` as an alternative
 README.md                 # User-facing docs.
@@ -220,6 +221,31 @@ session.
 
 No flags, no `agent_*` event handlers — fully passive until `/ext` is invoked.
 
+### `todo.ts`
+
+Unchanged copy of upstream `examples/extensions/todo.ts` from
+`pi-coding-agent` (the homebrew install at `/opt/homebrew/Cellar/...`).
+Provides the agent with a `todo` tool (actions: list/add/toggle/clear)
+and registers `/todos` for the user to inspect the list.
+
+**Why copied, not symlinked:** symlinking would point at
+`/opt/homebrew/Cellar/pi-coding-agent/<version>/libexec/...` which
+rotates on every brew upgrade — fragile. Copy keeps it stable; we lose
+upstream updates but gain reproducibility.
+
+**State persistence:** the agent stores todo state in tool result
+`details`, not an external file. Two useful properties: state survives
+`pi --continue`/`--resume` because it lives in the session JSONL, and
+`/fork` correctly forks the todo list along with the conversation.
+
+**Refresh from upstream when needed:**
+
+```bash
+cp /opt/homebrew/Cellar/pi-coding-agent/*/libexec/lib/node_modules/@mariozechner/pi-coding-agent/examples/extensions/todo.ts \
+   ~/src/src_local/pi-extensions/extensions/todo.ts
+# review diff, then commit
+```
+
 No framework. Manual:
 
 ```bash