ci: remove per-repo add-to-roadmap workflow (superseded by central sweeper)

The roadmap auto-add is now handled centrally by hypatia's roadmap-sync.yml
(one workflow, one secret). This per-repo pilot is redundant; with the
ADD_TO_PROJECT_PAT secret removed it would only fail on every new issue/PR.
Retiring it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath