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.
