[codex] Document Free and Pro billing

[roshanjonah]

What changed

• Documents Free, Pro, and Enterprise billing.
• Defines per-email billing units and exact rates.
• Documents Free's 2-mailbox and 10-recipient daily limits.
• Replaces Free allowance-request guidance with Pro upgrade guidance.
• Adds the billing release note and regenerates Management API and Postman output.

Checks

• mint validate
• mint broken-links
• npm run confidentiality:check
• npm run external-links:check
• npm run postman:check
• Rendered preview review with no console errors

Release gate

Publish these docs with the billing release, not before it.

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