chore: remove stale plan.md from repo root (#1510)

Session-scoped planning artifacts belong in session state, not in
the repo. This was a leftover from an earlier session.

Co-authored-by: danielmeppiel <danielmeppiel@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danielmeppiel