Add retry and circuit-breaker resilience around LLM HTTP calls (ak0)

Wrap the shared provider HTTP choke point (doJSONRequest in llm/util.go,
used by openai, openrouter, anthropic and ollama) with two resilience
patterns implemented with the standard library only:

- Retry with exponential backoff + jitter (resilience.go): 3 attempts by
  default, retrying transient network errors and retryable HTTP statuses
  (429 and 5xx). Client errors (4xx) and successes are returned
  immediately and never retried. Backoff waits are context-aware so
  cancellation and deadlines are respected; retried response bodies are
  drained and closed for connection reuse.

- Circuit breaker (circuitbreaker.go, own file as it has >3 methods):
  classic closed/open/half-open breaker that trips after 5 consecutive
  transient failures and stays open for a 30s cooldown, then allows a
  single trial probe. Only transient failures count; 4xx never trips it.
  A nil breaker is a valid no-op.

doJSONRequest now delegates to doJSONRequestResilient; the single-attempt
primitive is preserved as doJSONRequestOnce. Adds httptest-based unit
tests for retry/backoff/breaker logic with no real network calls;
new code coverage is >80%.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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