chore: redact WiFi PSKs and add NixOS build spec doc

Co-authored-by: aider (openrouter/openai/gpt-5.2) <aider@aider.chat>

docs/nixos_build_spec.md

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+# NixOS Build Spec (sam-4screen-desktop)
+
+## Disk / Boot (DO NOT BRICK WINDOWS)
+Target disk: /dev/nvme0n1
+
+KEEP (do not format):
+- /dev/nvme0n1p1 (EFI vfat) UUID=94CC-9DA4
+- /dev/nvme0n1p2 (MS reserved)
+- /dev/nvme0n1p3 (Windows NTFS) UUID=8294CD2994CD2111
+- /dev/nvme0n1p4 (Windows recovery NTFS) UUID=6E1AA32B1AA2EEF1
+
+WIPE + install NixOS here:
+- /dev/nvme0n1p5 (currently Ubuntu ext4 /) UUID=4e99904c-ce6f-4c3c-898a-e29cfc373489
+
+Bootloader:
+- systemd-boot, using the existing EFI partition (/dev/nvme0n1p1)
+
+## Networking
+- Use NetworkManager
+- Static IPv4 for wired: enp0s31f6 = 192.168.20.27/24
+- Gateway: 192.168.20.1
+- DNS: 192.168.20.35, 192.168.20.13
+
+## Data drive mount
+- Integral300 ext4 UUID=27febd74-20aa-4a3a-92c1-6fdd1ad7e88e
+- Mountpoint on NixOS: /data (preferred)
+
+## Desktop
+- Primary compositor: Niri
+- Display/login manager: TBD (recommend greetd)
+- GPU: Intel HD 630 + NVIDIA Quadro P620 (confirm which drives monitors)
+
+## Secrets policy
+- No API keys or WiFi PSKs committed to git