Use temp file + install pattern for Darwin compatibility

The previous pipe-based approach (printf | install /dev/stdin) doesn't
work reliably on macOS. Use temp file + install instead, which:
- Preserves exact contents of DEVENV_TASK_ENV (no shell expansion)
- Fixes file-owned-by-another-user problem (install recreates)
- Works on both Linux and macOS

See: nix-community/nix-direnv#647 (comment)

Assisted-By: claude-opus-4-5-20251101 via Claude Code

Author: tennox

src/modules/tasks.nix

@@ -210,7 +210,14 @@ in
         description = "Runs when entering the shell";
         exec = ''
           mkdir -p "$DEVENV_DOTFILE" || { echo "Failed to create $DEVENV_DOTFILE"; exit 1; }
-          echo "$DEVENV_TASK_ENV" | install -m 755 /dev/stdin "$DEVENV_DOTFILE/load-exports"
+
+          # Write to temp file first, then use install to atomically replace.
+          # This works even when the target is owned by another user (e.g., root),
+          # as install unlinks and recreates the file.
+          _tmp="$DEVENV_DOTFILE/load-exports.$$"
+          printf '%s\n' "$DEVENV_TASK_ENV" >"$_tmp"
+          install -m 755 "$_tmp" "$DEVENV_DOTFILE/load-exports"
+          rm -f "$_tmp"
         '';
       };
       "devenv:enterTest" = {