Studio Pro stores list-add values as expression strings, but MDL only accepted `add $Item to $List` and the AST kept only a bare item variable. Round-tripping a list-add action whose value is an attribute path or function call could therefore lose the original expression shape.

The grammar now accepts `add expression to $List`, the visitor stores the parsed expression while keeping the simple-variable compatibility field, and the builder writes the expression value back to the `ChangeListAction`. The existing variable-only form remains supported.

Tests cover parser and builder behavior for expression and simple-variable forms. The branch also adds a draft proposal, doctype fixture, quick-reference entry, and skill guidance; `mxcli check` on the fixture plus make build, make lint-go, and make test pass locally.