RFC-style discovery doc for the MCP server. Mirrors the OAuth discovery pattern — agents probe /.well-known/mcp.json to find the server URL + auth scheme without needing a pre-known path.

Two access paths:

1. Web tier (nicheangle.com/.well-known/mcp.json) — handled by the Next.js /api/mcp-discovery route at web/src/app/api/... Returns {web_origin}/api/mcp (Vercel rewrites to Fly internally).
2. Direct API (niche-api.fly.dev/.well-known/mcp.json) — this handler. Returns the bare API endpoint {api_origin}/mcp (no /api/ prefix; that's a web-tier rewrite path). The human-facing surfaces (token issue, docs) point at the configured FRONTEND_URL so agents that arrived via the API host still get correct links into the user dashboard.