User-supplied cost per 1M tokens for cost-based budget tracking

[jrswab]

Overview

Split from #19. Cost-based budgets (max_cost_usd) are deferred from the token budget feature because maintaining up-to-date pricing data per model is an ongoing maintenance burden.

Proposed Approach

Allow users to supply their own cost per 1M tokens in the agent TOML. Axe uses that rate to track spend against a configured budget — no pricing data maintained in the repo.

[budget]
cost_per_1m_tokens = 0.30   # user-supplied
max_cost_usd = 0.05

• Keeps the repo model-agnostic
• Gives users a way to track and cap by price
• No external pricing API or hardcoded model costs required

Notes

Deferred from #19 to keep the initial token budget implementation scoped and shippable.