feat(chapter-29): retry transient LLM provider failures

Chapter: 29 - Provider resilience.

Plain-English map:
Wrap LLM provider calls with exponential-backoff retries for temporary errors
like rate limits, timeouts, and provider outages.

Why it matters:
LLM providers occasionally fail for reasons the user cannot fix. A short,
careful retry can turn a temporary outage into a normal response.

Principle:
Classify external failures before retrying. Retry transient problems, not bad
requests or authentication errors.

Precedent borrowed:
The fork report's alternative-provider cluster and provider-abstraction work
across active forks.

Upstream base: willchen96/mike@d39f580.
Original local commit: 1417930.