docs: update stale dotfiles/nixos flake paths 📝

Stage 4f cleanup. The flake moved from ~/dotfiles/nixos/ to ~/dotfiles/
in 88c5139; docs and install scripts hadn't been refreshed. Point all
rebuild / flake references at the new root:

- AGENTS.md, README.md, server-quickstart.md, docs/server-installer-usb.md,
  docs/sunken-ship-wifi.md, nixos/readme.md — rebuild command paths.
- scripts/nixos-server-install.sh — auto-detect now looks for flake.nix
  at repo root (was nixos/flake.nix).
- scripts/post-install-provision.sh — first-rebuild hint path.

`nixos/hosts/<host>-hardware.nix` and friends stay where they are —
host-specific NixOS modules still live under nixos/; only the flake
entry-points + sops/ + vars/ + lib/ + modules/ + flake-modules/ moved.

nixos/readme.md rewritten to reflect the split (flake at root, per-host
modules under nixos/).

docs/sunken-ship-wifi.md

@@ -42,10 +42,10 @@ nix shell nixpkgs#wpa_supplicant -c wpa_passphrase "YOUR_SSID" "YOUR_PASSWORD"
 
 ## Rebuild (after changing Nix config)
 
-From the server (flake is in `nixos/`):
+From the server (flake is at the repo root):
 
 ```bash
-cd /etc/dotfiles/nixos && sudo nixos-rebuild switch --flake .#sunken-ship
+cd /etc/dotfiles && sudo nixos-rebuild switch --flake .#sunken-ship
 ```
 
 ## Verify