feat: complete and prove the authorization chain end to end

Implements step 2 (access-engine POST /v1/check) and wires the whole
GH-DEC-2026-003 sequence together, then proves it against a live throwaway
OpenBao rather than only unit-level fakes.

- decision_check.check_decision performs the PDP call; an unreachable or
  non-200 PDP raises, since silence is never permission.
- approval_consume.authorize_action coordinates steps 1 and 2 and returns an
  AuthorizedAction. Both steps build the same CheckRequest via a shared
  _expected_request, since two descriptions of the action cannot produce
  corresponding digests.
- apply_unreachable_engine_stance takes authorized=. The published map
  defines fail_closed as no side effect WITHOUT a durable decision record,
  so holding a validated one means the residue does not apply. Not a bypass:
  both steps must have succeeded and CAS consume still precedes OpenBao.
  Unconfigured still returns None and fails closed.

The end-to-end test caught one more instance of the cross-vocabulary bug: a
leftover comparison of the claim's binding.action against ours. The claim
says secrets.kv.destroy where we say destroy, so it would have failed against
every real claim. Removed; the tie is pdp_digest.

Integration coverage asserts PIP-then-PDP ordering, that consume is the last
step before the backend, and that an unreachable PDP, denied decision,
invalid claim, missing pdp_digest, consume conflict and action mismatch each
stop before OpenBao. 284 tests pass; production still fails closed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M65ovP3eiiPHubibvWs9mD

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 393550@bnt-lap001
Assistant-Session: 4bb359f9-1f12-4410-9e76-079cf23c82e4

src/secrets_engine/config.py

@@ -49,6 +49,8 @@ class Config:
     authorization_policy_package: str = ""
     authorization_policy_version: str = ""
     authorization_min_approvals: int = 1
+    pdp_url: str = ""
+    pdp_token_file: Path | None = None
 
     @classmethod
     def load(cls) -> "Config":
@@ -56,6 +58,7 @@ class Config:
         token_file = os.environ.get("SECRETS_ENGINE_APPROVAL_TOKEN_FILE", "")
         keycape_secret = os.environ.get("SECRETS_ENGINE_KEYCAPE_CLIENT_SECRET_FILE", "")
         jwt_login = os.environ.get("SECRETS_ENGINE_OPENBAO_JWT_LOGIN", "")
+        pdp_token = os.environ.get("SECRETS_ENGINE_PDP_TOKEN_FILE", "")
         return cls(
             catalog_dir=Path(os.environ.get("SECRETS_ENGINE_CATALOG", root / "catalog")),
             policy_dir=Path(os.environ.get("SECRETS_ENGINE_POLICIES", root / "policies")),
@@ -86,4 +89,6 @@ class Config:
             authorization_min_approvals=_positive_int(
                 os.environ.get("SECRETS_ENGINE_AUTHORIZATION_MIN_APPROVALS", "")
             ),
+            pdp_url=os.environ.get("SECRETS_ENGINE_PDP_URL", ""),
+            pdp_token_file=Path(pdp_token) if pdp_token else None,
         )