Add robust command availability checks to all shell scripts

- Add check for @defaultPackageName@-devenv in activateDevenv.sh
- Add check for uv command in initPython.sh
- Add check for git command in initProject.sh (with graceful degradation)
- Add checks for wget, sed, nix in updateDeps.sh
- Provide clear error messages when commands are not available
- Prevent scripts from failing with cryptic errors when tools are missing

Co-authored-by: dwinkler1 <22460147+dwinkler1@users.noreply.github.com>

templates/rde/scripts/activateDevenv.sh

@@ -2,6 +2,11 @@
 set -euo pipefail
 if [[ -f "devenv.nix" ]]; then
   echo "🚀 Activating devenv environment..."
+  if ! command -v @defaultPackageName@-devenv &> /dev/null; then
+    echo "❌ Command '@defaultPackageName@-devenv' not found."
+    echo "Ensure devenv is properly configured in your environment."
+    exit 1
+  fi
   exec @defaultPackageName@-devenv shell
 else
   echo "❌ No devenv.nix file found in the current directory."