Developer portal redesign

/developers is now a single sticky-TOC page on a warm cream surface with a terracotta accent. Quickstart walks integrators from key creation to first call to Stripe checkout in three steps. The Playground runs real requests against the public API with the key the integrator just created (stored locally only). API reference is grouped by domain (Search, Facility, Trust, Decision, Coverage, Billing, Health, Memory) with free-text filter. MCP install ships valid JSON for Claude Desktop, Cursor, Codex, npx, and the remote MCP URL — env vars now match the actual server (MCP_API_KEY / MCP_API_BASE_URL); a permanent drift checker keeps it that way. cmd+K opens a command palette indexing every section, route, and resource. New backend endpoints GET /v1/developer/sections and GET /v1/developer/quickstart let MCP and llms.txt consumers pull the same source-of-truth without scraping HTML. New light-theme OG and Twitter images for /developers/*. The dark global chrome is suppressed under /developers via an x-pathname middleware hook plus a .portal-theme wrapper. 16 new pytest invariants lock llms.txt, public-contract, sitemap, robots, JSON-LD, and MCP-package-name consistency.