🤖 chore: cache nix devshell PATH for mux bash tools

---
_Generated with [`mux`](https://github.com/coder/mux) • Model: `openai:gpt-5.3-codex` • Thinking: `xhigh` • Cost: `$1.22`_

<!-- mux-attribution: model=openai:gpt-5.3-codex thinking=xhigh costs=1.22 -->

Author: ThomasK33

.mux/tool_env

@@ -2,6 +2,61 @@
 # Sourced by Mux before every bash tool invocation.
 # Docs: https://mux.coder.com/hooks/tools
 
+mux_use_nix_devshell_path() {
+  # Keep cache file workspace-specific to avoid collisions between parallel workspaces.
+  local workspace_cache_tag="${MUX_WORKSPACE_ID:-${MUX_WORKSPACE_NAME:-${PWD}}}"
+  workspace_cache_tag="$(printf '%s' "${workspace_cache_tag}" | tr -cs 'A-Za-z0-9._-' '_')"
+
+  local cache_file="/tmp/mux-nix-path-${workspace_cache_tag}"
+
+  # Only attempt activation for flake-based projects with nix installed.
+  if [[ ! -f "flake.nix" ]]; then
+    return 0
+  fi
+  if ! command -v nix >/dev/null 2>&1; then
+    return 0
+  fi
+
+  local should_refresh=0
+  if [[ ! -s "${cache_file}" ]]; then
+    should_refresh=1
+  elif [[ "flake.nix" -nt "${cache_file}" ]]; then
+    should_refresh=1
+  elif [[ -f "flake.lock" && "flake.lock" -nt "${cache_file}" ]]; then
+    should_refresh=1
+  fi
+
+  if [[ "${should_refresh}" -eq 1 ]]; then
+    local tmp_cache_file
+    tmp_cache_file="$(mktemp "${cache_file}.tmp.XXXXXX")"
+
+    if nix develop -c bash -c 'echo "$PATH"' >"${tmp_cache_file}" 2>/dev/null; then
+      if [[ ! -s "${tmp_cache_file}" ]]; then
+        rm -f "${tmp_cache_file}"
+        echo "assertion failed: nix develop returned an empty PATH" >&2
+        return 96
+      fi
+      mv "${tmp_cache_file}" "${cache_file}"
+    else
+      rm -f "${tmp_cache_file}"
+      if [[ ! -s "${cache_file}" ]]; then
+        echo "assertion failed: failed to populate nix devshell PATH cache: ${cache_file}" >&2
+        return 96
+      fi
+    fi
+  fi
+
+  if [[ -s "${cache_file}" ]]; then
+    export PATH="$(<"${cache_file}")"
+    return 0
+  fi
+
+  echo "assertion failed: nix devshell PATH cache file missing or empty: ${cache_file}" >&2
+  return 96
+}
+
+mux_use_nix_devshell_path
+
 run_and_report() {
   local step_name="${1:-}"
   if [[ -z "${step_name}" ]]; then