Sites for agent output: publish reports/artifacts with signed provenance

[brianorwhatever]

From docs/business-plan.md §7 Phase 3. Ties all three product surfaces into one loop: agent does work (lists) → produces artifact (Sites) → with proof (VCs).

• Agent API endpoint to create/replace a Site's HTML (extend convex/siteActions.ts surface to API-key auth with a sites:write scope)
• Link a Site to the mission run that produced it (run artifacts already support url type)
• Use cases: agent-published reports, dashboards, status pages on *.boop.ad with signed lineage
• Dogfood first: boop's own status page and blog published this way

Timing note: Phase 3 — after Phase 1–2 traction.