fix(pricing): fast multiplier for the bare gpt-5.6 alias id

ccusage maps gpt-5.6 to gpt-5.6-sol (pricing_alias) before multiplier
and pricing lookups; models.dev catalogs the bare id as its own entry
with sol's rates, so it resolved exactly here and skipped the alias's
2x fast multiplier — fast usage recorded under gpt-5.6 billed at half
of what ccusage bills. Also document that git-ai usage's flat estimate
intentionally keeps Claude-shape cache defaults for every tool; the
TokenUsage event pipeline is the authoritative dollars.

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