Final G9 step. The schema no longer creates current_room or
room_seq_json on fresh installs, and existing prod schemas drop them
via purgeLegacyZoneRunColumns() once the operator flips
GOGOBEE_BRANCHING_PURGE=1.
- CREATE TABLE dnd_zone_run loses both columns; comment updated to
point at G1's columnMigrations for the graph-mode columns.
- cleanup_g9.go mirrors the L5 idempotent-purge shape: ALTER TABLE
DROP COLUMN per legacy column, "no such column" treated as already
done, default-off.
- Wired into adventure Init alongside purgeLegacyAdvCharacterTable.
- Plan doc marks G9 complete.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
