feat(compare): add counterfactual pricing

[ozymandiashh]

Summary

This adds a pricing-only what-if mode to codeburn compare: codeburn compare --reprice <model>. The normal interactive model comparison stays unchanged, but users can now ask how their recorded usage would have priced if every call had used a different model.

The calculation keeps the original usage shape: input tokens, output tokens, cache creation, cache reads, web-search requests, and fast-mode flags are reused exactly as recorded. Only the model pricing table changes. That makes the output useful for budget planning while keeping the scope honest: it does not claim to simulate model quality, different reasoning behavior, or shorter/longer outputs.

What changed

• add codeburn compare --reprice <model> as a non-interactive what-if pricing mode
• add --json for scripted/reporting workflows when --reprice is used
• validate the target model before parsing sessions, using the existing pricing and alias lookup
• report actual spend, what-if spend, savings or added cost, and percent difference
• surface the largest session impacts, plus project and source-model breakdowns
• skip <synthetic> calls so provider bookkeeping entries are not repriced as real API calls
• preserve fast-mode pricing semantics for the target model
• document the new workflow and add an Unreleased changelog entry

Example

codeburn compare --reprice claude-sonnet-4-5
codeburn compare -p week --provider claude --reprice gpt-5.3-codex
codeburn compare --reprice gpt-4o-mini --json

Output shape

The text output is meant for quick budget decisions:

• total actual vs what-if spend
• net savings or added cost
• top session impacts sorted by absolute difference
• project-level impact
• source-model impact

The JSON output returns the same structure in USD fields so downstream scripts do not have to parse localized currency strings.

Validation

• npx vitest run tests/reprice.test.ts
• npx vitest run
• npm run build
• node dist/cli.js compare --help
• node dist/cli.js compare --reprice gpt-4o-mini -p today --json
• node dist/cli.js compare --json

Note: full npx tsc --noEmit is still blocked on origin/main by existing src/providers/copilot.ts type errors; this branch does not touch that provider.