Karat prices over MCP + anonymous signup nudge

New get_carat MCP tool: per-gram gold (XAU) prices by karat purity (24k through 10k) across 13 currencies, free on every tier. The MCP server is now 7 tools. Separately, anonymous (no-key) responses on /v1/spot/{symbol} and /v1/prices now carry a Link header and an X-Goldprice-Notice header pointing to /onboarding, so keyless callers can discover the free API-key signup. Keyless access stays open and the JSON body is unchanged.