feat(install): #59 NVIDIA commented system.nix guidance

When hardwareProfile includes common-gpu-nvidia, emit a comment-only
plain NixOS block (PRIME / powerManagement / open-module) in the
installer region of system.nix — same pattern as ROCm/NPU opt-ins.
HARDWARE.md §6 product direction marked shipped. Close #59.

Verified: V0 (patch-template smoke + negative + bash -n).

agent/BACKLOG.md

@@ -95,15 +95,6 @@ the Wi‑Fi card.” Does **not** rewrite the flake unless a later `--apply`
 is explicitly designed. Cost: extract script + package + man/README
 pointer; V0–V1.
 
-### 59. Installer: NVIDIA commented system.nix guidance
-Detection already adds `common-gpu-nvidia`; there is no
-`nomarchy.hardware.nvidia.*` and hybrid/PRIME is wiki territory. When
-NVIDIA is detected, emit a **commented** block in generated `system.nix`
-(same style as ROCm/NPU) pointing at PRIME / powerManagement /
-open-module and docs/HARDWARE.md §6 — not a full second stack. Cost:
-install script strings; V0 + doc. Prefer this over first-class NVIDIA
-options (`[big]` stays PROPOSED).
-
 ### 60. Battery / powerprofile: name-agnostic battery scan
 `power.nix` oneshot and menu/waybar globs use
 `/sys/class/power_supply/BAT*`. `nomarchy-battery-notify` correctly