Add retry mechanism driven by pi settings.json retry.provider config
(timeoutMs, maxRetries, maxRetryDelayMs).
HTTP-level retries handle 429/5xx with exponential backoff and jitter,
respecting Retry-After headers (seconds and HTTP-date formats).
Stream-level retries handle cases where the API returns 200 OK but
sends an error event in the stream body. Retries only when no content
has been emitted yet.
Per-attempt timeout via AbortController with automatic retry. Clean
abort propagation through the retry loop.
Document the incremental reasoning-delta streaming, live display pricing,
and dynamic model fetch in CHANGELOG.md. Add a Pricing section to
README.md explaining the MODEL_COSTS overlay and how to update prices.
