Consolidate LLM timeout/retry/breaker values into policy package

The per-request HTTP timeouts (30s chat, 120s research), retry policy
(attempts, backoff, jitter), and circuit-breaker tuning (threshold,
cooldown) were previously scattered as bare literals across each provider
constructor and resilience.go/circuitbreaker.go, making the operational
policy hard to discover and prone to drift.

Introduce internal/llm/policy with documented named constants as the single
source of truth, and reference them from all provider constructors, the
default retry policy, and the shared circuit breaker. Update comments to
explain the policy and reasoning. Add tests validating the values and their
internal consistency.

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

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