Document X-AISA-Cost-Mode quote in public OpenAPI

[eddiearc]

Problem

X-AISA-Cost-Mode: quote is implemented by AIsaServices and documented in Agent Quickstart, but it is absent from the published https://aisa.one/openapi.yaml. An API consumer that reads only endpoint/OpenAPI contract cannot discover quote mode or the cost_estimate response.

Scope

Document the quote capability in the public API contract without claiming universal support:

• header: X-AISA-Cost-Mode: quote
• response: cost_estimate, including estimate_kind, estimated_cost_micros_usd, optional max_cost_micros_usd, and may_exceed_estimate
• semantics: quote validates/estimates but does not execute, reserve funds, authorize spend, or lock price
• eligibility: only backend quote-aware execution routes support it; unsupported operations return cost_quote_unsupported

Acceptance

1. The published aggregate OpenAPI exposes one reusable, machine-readable quote header/response contract.
2. The rendered API Reference explains how to use it and the no-execution semantics.
3. The contract does not mark all operations as quote-capable unless route-level support is generated from an authoritative backend inventory.
4. The docs source/aggregate generation check passes; no backend or billing behavior changes are part of this issue.

Context

Read-only audit of AIsaServices origin/main@a7b861d4 found global header parsing plus explicit quote-aware/quote-rejected route policy. The capability is real, but OpenAPI publication does not currently derive it from that policy.