refactor(token-usage): one tier-rate rule for both cost shapes

codex_cost_usd re-implemented the above/base rate fallback as a nested
tuple construction, duplicating in expanded form what claude_cost_usd
expresses through its rate closure; a shared tier_rate helper now
backs both, so the fallback rule cannot drift between agents
(behavior verified identical in all four explicit x long-context
combinations). entry_cost_micro_usd becomes test-only — production
stores the full EntryPricing, and a parallel entry point invited
callers to drop the recorded long-context/catalog facts. The
pricing_catalog_id doc now states that the id pins the fetched data
only; effective rates are attributable through it together with the
git_ai_version attribute (override tables are compile-time constants).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>