Pluggable expression languages

A separate expression language should be supported, like inside `$(..)` or whatever. Users should be able to use whatever expression language they like.

The issue names no files, tests, or entry points. First clarify the expression syntax, supported language boundary, and integration expectations; then inspect the shellkit architecture to identify where expressions are parsed and evaluated. Done means a documented extension mechanism and tests demonstrating a separate expression language inside the chosen syntax.

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