Phase 6c — Public ACO Registry + Wiki Flows (new Linear ticket)

Phase 6c turns the hosted ACP MCP server at `mcp.atomiccontentprotocol.org` into a public, content-addressable ACO registry with lookup, publish, find-related, and stats tools, while Stacklist MCP dual-writes enrichments to both its private Supabase database and the public registry with a read-through fallback chain. It also introduces DWIM behavior for `enrich_url` on stacklist.com URLs, where card/stack owners get in-place enrichment updates while non-owners contribute enrichments only to the public registry without modifying the owner's private data—establishing a wiki-like collaborative dynamic.