mdx skill: document the open <Pkg> convention for in-package examples
Captures the rule the recent ocaml-claude .mli changes converged on:
when a .mli lives inside package `Foo` and uses many `Foo.X` /
`Foo.Y` references, `open Foo` at the top of the block reads better
than peppering the prefix on every call site.
Distinguishes packages whose top-level module groups several
sub-modules a typical example uses together (Claude, Irmin) -- where
the open is helpful -- from single-dominant-module packages
(Toml, Sse) where the qualified form is already short.