Implement GH-DEC-2026-003 consume-before-OpenBao PEP gate

Every live privileged production handler CAS-consumes through
approval-engine before OpenBao. Conflict, unavailability, or a missing
binding fail closed. Live production remains disabled until the durable
decision record is served.

Record kings-guard assent on the secret-use evidence contract.

Assistant: grok
Assistant-Session: 01a05f07-ae72-7781-9fcb-19efd61add00

docs/approval-consumption.md

@@ -0,0 +1,49 @@
+# Approval consumption (PEP)
+
+Status: engine consumer implemented; live production remains fail-closed
+until a durable consume binding is served.
+
+Normative protocol: `gate-house/docs/contracts/approval-consumption.md`
+(`GH-DEC-2026-003`). Implementation surface:
+`approval-engine/docs/approval-consumption.md`. This document records how
+secrets-engine, as the PEP for OpenBao writes, consumes that protocol. It
+does not redefine it.
+
+## Sequence
+
+```text
+1. PIP  GET  /v1/approvals/{id}/claim
+2. PDP  access-engine Check / decide          → ALLOW
+3. PEP  POST /v1/approvals/{id}/consume       → CAS
+4. PEP  OpenBao call                          → only after consume succeeds
+```
+
+Every live privileged production handler passes `_require_lane_approval`,
+which calls `require_production_consume` before `OpenBaoClient.resolve`.
+Dry-run and `plan` do not consume. Build/test remain fail-open relative to
+approval-engine. The three-factor unsafe-demo exception is not a consume
+path.
+
+## Fail closed
+
+| Condition | Result |
+| --- | --- |
+| No served consume binding | refuse; no OpenBao |
+| Missing `SECRETS_ENGINE_APPROVAL_URL` or token file | refuse; no OpenBao |
+| HTTP 409 / different digest | refuse; no OpenBao |
+| Same digest after consume | idempotent success; OpenBao may proceed |
+| 401/403/404/503/unreachable | refuse; no OpenBao |
+| Side effect fails after consume | approval is spent; no unconsume |
+
+The consume response is mutation evidence (`status=consumed` plus the
+presented digest). It is not a permission. Evidence records approval id,
+digest, idempotence, and consumed-at only. No token, secret, or accessor.
+
+## What this does not do
+
+- It does not enable live production. Unreachable-engine stance still
+  fail-closes production until an access-engine decision record is served
+  (`SECRETS-WP-0007-T04` / `SECRETS-WP-0008-T02`).
+- It does not render or cache an authorization decision.
+- It does not infer consumption from a decision record or from local
+  evidence.