fix(.envrc): use uv -> layout uv

Direnv has no built-in `use_uv`; the `use uv` line in 219f045 silently
errored out (`use_uv: command not found`) and the shell ended up with
an unactivated `.venv`. The matching upstream proposal is `layout_uv`
in direnv/direnv#1329 (open, maintainer-approved). Install that
function (verbatim, plus a small auto-sync-on-pyproject-change block)
in ~/.config/direnv/direnvrc on the local machine and switch the
.envrc verb here to match. When #1329 ships in a direnv release, the
custom direnvrc function can stay (strict superset) or be removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.envrc

@@ -2,6 +2,6 @@
 
 PATH_add bin
 
-use uv
+layout uv
 
 source_env ~/.local/share/direnv/bundlewrap