chore(cost): mark canonical pricing table and cross-reference mirrors

estimate_cost_usd is necessarily duplicated: two self-contained `python -c`
stream-parser templates (agent.py, codex.py) must not import archon, and the
dashboard copy is TypeScript. Rather than re-architect the wrappers, label
state/cost.py as canonical and point each mirror at it so the four don't drift
again (the deepseek-reasoner mispricing came from exactly such a drift).