Generate Clojure functions on demand via the LLM, caching the generated
source so each function is only generated once.

(defextend (update-packages []) "Update all outdated dependencies")

defines `update-packages` immediately, right there: the body is written by
the LLM on the spot, persisted to `generated-file`, and eval'd into
`hyperphor.ellum.generated`. On a later call to `defextend` with the same
name -- a re-eval of the same form, or a future process once
`generated-file` has loaded -- the cached definition is reused and the LLM
isn't consulted again.

Ported from `claude-extend` (mt-elisp), which shells out to the `claude`
CLI since Emacs has no LLM client of its own; here we just call
`hyperphor.ellum.core/query` directly.