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/JOURNAL.md

@@ -17,6 +17,18 @@ Template:
 
 ---
 
+## 2026-07-10 — #59 NVIDIA commented system.nix guidance
+- **Task:** NEXT #59 — when `common-gpu-nvidia` is selected, emit
+  commented PRIME/power/open-module guidance in installer-generated
+  system.nix (ROCm/NPU pattern).
+- **Did:** `has_nvidia` from HW_PROFILES → patch JSON; `build_installer_region`
+  emits comment-only plain NixOS block + HARDWARE.md §6 pointer. Docs §6
+  product direction marked shipped. Closed #59.
+- **Verified:** **V0** — patch-template smoke with nvidia true (region
+  contains hardware.nvidia.* comments + profile in flake); negative
+  (nvidia false) has neither; `bash -n` install script.
+- **Next suggestion:** #60 BAT*-agnostic battery scan.
+
 ## 2026-07-10 — #62 whole-swap CSS self-containment CI gate
 - **Task:** NEXT #62 — cheap gate that waybar.css defines every @color it
   references (would have caught neon-glass) + preview/backgrounds present.