feat: the offline_access mandate

The standing authority the autonomous runs act under: an offline refresh token
issued by a new classic-models-mandate client for the user who authorised the
agent, stored in Vault, redeemed at trigger time.

The mandate client holds no domain audience. It cannot reach data; it can only
say who authorised whom. Direct access grants on it is a lab shortcut, noted as
such — mandating an agent is a decision a person takes, and a password grant
records no consent where the authorization code flow would.

Verified end to end: refresh gives sub=pm-user azp=classic-models-mandate with
the agent in the audience, the agent exchanges it to azp=itself and
aud=mcp-catalog, and the MCP call answers 200. sub stays the person who
mandated, azp says which agent acted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

deploy/agent-secrets.yaml

@@ -22,3 +22,9 @@ spec:
       remoteRef:
         key: keycloak/classic-models-realm
         property: KC_SECRET_CLASSIC_MODELS_AGENT_APPROVISIONNEMENT
+    # Redeems a mandate. Held by the workflow rather than by the adapter because
+    # it is the workflow that acts as the agent.
+    - secretKey: MANDATE_CLIENT_SECRET
+      remoteRef:
+        key: keycloak/classic-models-realm
+        property: KC_SECRET_MANDATE