Outcome-based billing — meter successful agent actions for usage-based pricing

[shreyanshjain7174]

Per-action metering with integration points for OpenMeter, Stripe Metering, internal chargebacks.

Acceptance:

• BillableEvent emitted on successful agent action
• OpenMeter integration (HTTP push)
• Stripe Metering integration (HTTP push)
• Internal chargeback CSV export
• Configurable via Helm values