fix: sync staged workflows and Ask AI billing docs

[cursor]

Findings

Wrong

• Correct gt translate behavior when staged mode has no versions: it exits successfully without downloading files.
• Document Ask AI's at-cost token metering, LCU conversion, shared free allowance, and Enterprise billing-account requirement.

Polish

• Clarify that free credits use a rolling 30-day window and that workflows are charged from metered usage after any reservation.

Verification

• Fresh-installed gt@2.16.0 and gtx-cli@2.16.0; probed the published flags and empty staged-download result.
• Ran all content validator tests, type checking, structural/link/reference/HTML/callout checks.
• Built all 459 static content pages successfully.