docs: add billing preview and Goal pricing guidance

[yechang15]

Summary

• Add Billing as a top-level documentation tab, explicitly marked as an upcoming pricing preview. Current billing remains unchanged.
• Document public Model and Carrier rates, US/CA domestic coverage, preparation usage, and the fully waived experimental Task Success Fee.
• Highlight Goal half-price pricing with a responsive 30-second stacked cost comparison, savings amount, and individual fee-rate tooltips.
• Keep concise Calls and Goal Runs API/SDK guidance below the chart, and add a prominent Goal recommendation to the Calls guide.
• Preserve historical changelog content unchanged.

Validation

• Full pnpm run validate passed with Python 3.13.
• All 27 Playwright tests passed, including light/dark themes, desktop/mobile layouts, keyboard/touch tooltips, pricing calculations, and guide links.
• git diff --check passed.

Scope

Documentation and docs-site UI only. No backend billing implementation, API contract, dependencies, or deployment workflows are changed.

No release needed: this private documentation site does not require a package version bump or changeset.