docs: publish live billing guidance

[yechang15]

Summary

• Remove the Billing preview banner, Coming soon badge, and Preview labels from the comparison chart, tooltips, and exported content now that the billing model is live.
• Update Billing and Calls guidance to describe current pricing instead of upcoming pricing.
• Remove unused preview styles while preserving pricing calculations, API guidance, and the fully waived experimental Task Success Fee note.
• Leave historical changelog content unchanged.

Validation

• Full pnpm run validate passed, including all 31 Playwright tests.
• Verified desktop/mobile layouts, fee breakdown tooltips, and Markdown exports.
• git diff --check passed.

Release

No release needed: documentation-site changes only; no package version bump or changeset required.