* fix(nix): make extraPackages actually work — wire into per-user profile
#17030 deprecated extraPackages because it only set the systemd service
PATH, which the terminal backend's login-shell snapshot discards.
Instead of deprecating, fix it: set users.users.${cfg.user}.packages
so NixOS builds a per-user profile at /etc/profiles/per-user/hermes/bin.
This path is included in PATH by /etc/set-environment, which the login
shell sources, so the terminal backend's snapshot picks it up.
One line of actual logic:
users.users.${cfg.user}.packages = cfg.extraPackages;
Verified in a NixOS VM test: su - hermes -c 'which hello' resolves
to /etc/profiles/per-user/hermes/bin/hello.
Reverts the deprecation warning and docs changes from #17030, restores
extraPackages as the recommended way to give the agent extra tools.
Container mode is unaffected — extraPackages was always native-only
(the systemd path line is inside !cfg.container.enable).
* nix: clarify additive merge semantics for extraPackages user profile
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Co-authored-by: Siddharth Balyan <daimon@noreply.github.com>
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