Cuts v0.15.0 — runtime JSON provider config (#117) and per-provider
request queue with telemetry (#115), plus example provider JSONs and
guide updates (#116) and the initial OpenRouter built-in (#113).

Also includes a release-blocking precedence fix: Config.req_llm_provider_tag/1
was checking the built-in tag map before the registry, so a JSON-defined
provider whose name collided with a built-in (in practice :openrouter)
had its spec's req_llm_provider_tag silently ignored. The registry spec
is now authoritative, with the built-in map as fallback.

Promotes the (already-populated) CHANGELOG Unreleased section to v0.15.0
and registers the new public modules (ProviderRegistry, ProviderSpec,
ModelDiscovery, RequestQueue + Supervisor, Telemetry) in the hexdocs
groups_for_modules.