feat: load model prices from JSON files

[1cbyc]

Summary

• Add --prices-file support for count, budget, and models
• Load JSON price tables that add or override model pricing
• Add TOKENMETER_PRICES_FILE for reusable default price-file configuration
• Keep explicit --prices-file values higher priority than the environment variable
• Validate price-file schema and surface bad files as CLI input errors
• Document the custom price-file format and environment variable

Test Plan

• uv run pytest
• uv run ruff check .

Fixes #2