Removes the webinar-dash plugin, dashboard.md, wiki/script-coverage.md,
the spec and plan under docs/, and the git infrastructure added for the
work. CLAUDE.md, index.md and log.md are restored to their pre-dashboard
content, so Workflow D, the #coverage tag and the sync log entry are gone.
The vault now matches its state at the start of the dashboard work. The
work remains reachable at the dashboard-work-archive tag.
