fix(providers): harden static credential placement

crates/openshell-core/README.md

@@ -58,6 +58,25 @@ router agree on provider defaults. Profiles define:
 Do not duplicate provider-specific inference behavior in callers. Add shared
 behavior here, then consume it from the gateway, sandbox, and router.
 
+## Static Credential Resolution
+
+Static credential bindings retain endpoint authorization. The gateway includes
+static `ProviderProfileCredential` auth metadata in the existing runtime
+credential map under `static-env:<ENV_KEY>`. Provider state extracts those entries
+before storing dynamic token grants and compiles placement locally alongside the
+matching static binding. Missing metadata defaults to the `Authorization` header;
+malformed reserved metadata or an invalid selected placement fails closed
+atomically. The same header default applies to request rewriting with standalone
+resolver snapshots.
+The credential's `auth_style` selects the placement; unused fields for other
+styles are ignored and do not authorize additional locations.
+
+Request rewriting supplies the actual header name, query key, or path slot.
+Header authorization is independent of authentication scheme, including Basic.
+Explicit body/WebSocket rewrite policies and trusted signing retain their
+existing permissions. Existing placeholder and stable-handle derivation remain
+unchanged. Protobuf message shapes and service interfaces remain unchanged.
+
 ## Middleware Contracts
 
 Built-in supervisor middleware identifiers, host-selector matching, and pure