chore: remove stray cpo/specs tree introduced by lode qufbiyo2
The structured-message-history decision log addendum landed in the solstone
repo by mistake; the authoritative copy is in the extro Org at
cpo/specs/shipped/generate-structured-message-history.md. Removing the stray
file and its parent directories keeps solstone clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>