* feat(coding-agent): add before_provider_headers extension hook
Extensions can already rewrite the request payload through before_provider_request, but there is no way to adjust the outgoing HTTP
headers of a provider call. This hook fills that gap for cases like request tracing, session correlation, or tenant routing.
Handlers mutate the headers map in place - a null value deletes a header - and the return value is ignored, so a handler cannot accidentally drop auth or attribution headers by
forgetting to spread.
* docs(coding-agent): document before_provider_headers extension hook
Add the before_provider_headers section and lifecycle-diagram entry to
extensions.md; drop the now-superseded proposal file.
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