Remove dead workshops#summary action

[maebeale]

🤖 suggested review level: 3 Read 📖 pure dead-code removal; verified unreachable before deleting

Why

Follow-up to #2307. Removing users.agency_id left workshops#summary as the column's last (vestigial) reader, so I checked whether the action is reachable. It isn't — it's dead three ways over:

• No route — resources :workshops only nests comments; there is no summary route.
• No view — there's no workshops/summary.html.erb, so it couldn't render.
• No policy rule — authorize! :workshop, to: :summary? targets a summary? that no policy defines, so the action would raise NoMethodError on its first line if ever reached.
• No tests reference it.

What

• Removed the summary action.
• Removed its private build_report helper (local to this controller — only summary called it; ReportsController has its own).
• Removed User#organization_workshop_logs (the 3-arg one) — only summary called it. organization_monthly_workshop_logs stays; ReportsController still uses it.

Not touched

• monthly_reports/_form.html.erb still renders _combined_workshop_log_summary with @combined_workshop_logs, which is now simply never assigned (it was already always-nil in prod). That's legacy monthly-reports territory — separate cleanup.