extraPackages adds packages to the systemd service PATH, but the
terminal backend's login-shell snapshot rebuilds PATH from NixOS system
profiles, so tools added via extraPackages are invisible to terminal
commands, skills, and cron jobs — the entire use case.
Changes:
- Mark the option description as deprecated with explanation
- Emit a NixOS warning when extraPackages is non-empty, including a
ready-to-paste environment.systemPackages replacement
- Update docs: quick-reference table, plugin example, and options
reference all point to environment.systemPackages
The option still functions (non-breaking) so existing configs keep
working while users migrate.
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