Add Cloudflare provider

[c99e]

Summary

Add Cloudflare as a supported GraphQL provider. Cloudflare exposes a GraphQL Analytics API at a fixed endpoint, authenticated via API Token (Bearer). This is the simplest auth model so far — no token exchange, no per-account URL — making it a clean addition once multi-provider support (#5) is in place.

Acceptance criteria

• A CloudflareProvider implements the GqlProvider interface
• The provider is detected when CLOUDFLARE_API_TOKEN is set
• The endpoint is https://api.cloudflare.com/client/v4/graphql
• Auth header is Authorization: Bearer <token>
• When CLOUDFLARE_API_TOKEN is missing, the provider is not detected — it does not block other providers
• The error message when no providers are configured lists Cloudflare and its required env var alongside existing providers
• reset() is a no-op (no cached state beyond the static token)
• README documents the Cloudflare configuration section and env var

Testing guidance

• Unit tests for config validation (missing env var throws, present env var constructs provider)
• Unit tests for endpoint and headers output
• Unit tests for detectProvider recognizing Cloudflare alongside existing providers
• Follow the existing LinearProvider tests as a model — the auth model is similarly simple

Dependencies

• Depends on Multi-provider support #5 (Multi-provider support)

Out of scope

• API Key + Email (legacy auth) — only API Token is supported
• Cloudflare-specific query helpers or analytics shortcuts
• Multiple Cloudflare accounts in one session