feat(32.1-01): publish one MCP scope list and call context

- pact gains MemberTokenScopeCatalog, MCPAdvertisedScopes and MCPCall
- an empty MCP ScopesSupported resolves to the five fixed names including backend
- pier copies advertised scopes and sets MCPCall beside WithDispatch

modules/pact/capabilities.go

@@ -817,11 +817,14 @@ type HasTokenScopes interface {
 // member token screens show. Label and Description are phrase keys
 // (containing "::") or literal text. It is the frontend counterpart of
 // TokenScope and carries no permission patterns or method filter: the route
-// middleware alone decides what a member scope allows.
+// middleware is what enforces a member scope. Implies is only "", "read" or
+// "write", for a later OAuth MCP check; the zero value means the scope is
+// not implied. This package does not perform that check.
 type FrontendTokenScope struct {
 	Name        string
 	Label       string
 	Description string
+	Implies     string
 }
 
 // HasFrontendTokenScopes is implemented by plugins that declare member