fix(nix): enable nix command on all machines

- Use bash instead of POSIX sh (process substitution fails on dash)
- Create persistent ~/.config/nix/nix.conf with experimental-features
- Use temp file instead of <() for portability

Author: jul-sh

nix/install.sh

@@ -1,5 +1,5 @@
-#!/bin/sh
-# nix/install.sh - POSIX compliant Nix installer wrapper
+#!/usr/bin/env bash
+# nix/install.sh - Nix installer wrapper
 
 set -eu
 
@@ -16,27 +16,48 @@ source_nix_profile() {
     die "Nix profile not found. Restart terminal and re-run."
 }
 
+ensure_nix_conf() {
+    local conf_dir="$HOME/.config/nix"
+    local conf_file="$conf_dir/nix.conf"
+    local required_line="experimental-features = nix-command flakes"
+
+    mkdir -p "$conf_dir"
+    if [ ! -f "$conf_file" ] || ! grep -q "experimental-features" "$conf_file"; then
+        echo "$required_line" >> "$conf_file"
+    fi
+}
+
 install_nix() {
     if command -v nix >/dev/null 2>&1; then
+        ensure_nix_conf
         return 0
     fi
 
     echo "Installing Nix..."
+
+    # Create temp file for extra config (process substitution not portable)
+    local extra_conf
+    extra_conf=$(mktemp)
+    echo "experimental-features = nix-command flakes" > "$extra_conf"
+    trap "rm -f '$extra_conf'" EXIT
+
     if [ "${NO_SUDO:-}" = "1" ]; then
         # Single-user mode only (no sudo)
-        if ! curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install | sh -s -- --no-daemon --yes --nix-extra-conf-file <(echo "experimental-features = nix-command flakes"); then
+        if ! curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install | sh -s -- --no-daemon --yes --nix-extra-conf-file "$extra_conf"; then
             die "Nix installation failed"
         fi
     else
         # Try official installer with multi-user mode first
-        if ! curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install | sh -s -- --daemon --yes --nix-extra-conf-file <(echo "experimental-features = nix-command flakes"); then
+        if ! curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install | sh -s -- --daemon --yes --nix-extra-conf-file "$extra_conf"; then
             # Fallback to single-user mode
-            if ! curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install | sh -s -- --no-daemon --yes --nix-extra-conf-file <(echo "experimental-features = nix-command flakes"); then
+            if ! curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install | sh -s -- --no-daemon --yes --nix-extra-conf-file "$extra_conf"; then
                 die "Nix installation failed"
             fi
         fi
     fi
+
     source_nix_profile
+    ensure_nix_conf
 }
 
 install_nix