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/).

scripts/post-install-provision.sh

@@ -57,5 +57,5 @@ cryptsetup close crypted 2>/dev/null || true
 echo ""
 echo "=== Done! Remove USB and reboot. ==="
 echo "After unlocking LUKS, SSH in: ssh danny@${HOSTNAME}"
-echo "Then: cd /etc/dotfiles/nixos && sudo nixos-rebuild switch --flake .#${HOSTNAME}"
+echo "Then: cd /etc/dotfiles && sudo nixos-rebuild switch --flake .#${HOSTNAME}"
 echo "Commit ${HOSTNAME}-hardware.nix from the USB back to the repo."