The roadmap diagrams referenced issue numbers (#1, #8-#13, #43-#55) that no
longer exist on Codeberg: the GitHub to Codeberg migration reset this repo's
issue tracker, and those numbers were never re-created. scripts/check-roadmap.sh
existed only to keep that roadmap in sync with GitHub epic-labeled issues via
gh, which can't resolve a Codeberg/Forgejo repo anyway, so the gate had been a
silent no-op since the move. Removed the roadmap section, the gate script, its
two call sites, and its now-pointless rule doc, and pointed the remaining
work at the real, live issues (#4, #5, #6, #7, tracked under epic #8). Also
fixed two leftover Mihaela/PureYAML references in docs/RELEASE.md.
