[ENHANCEMENT] Expose an environment variable to let shells detect the embedded terminal

When this editor launches a terminal, there's currently no way for shell startup files (.zshrc, .bashrc, etc.) to know that they are running inside the editor rather than a dedicated terminal emulator. This makes it hard to conditionally adjust startup behaviour. For example, in small embedded terminals users may want to **skip heavier tooling** (e.g. terminal multiplexers like zellij, banners, long-running startup scripts), while keeping them enabled in a normal terminal. Other editors already solve this by setting environment variables. For example vscode sets: ``` TERM_PROGRAM=vscode TERM_PROGRAM_VERSION=<version> ``` This allows a simple gate in shell configs, e.g. ```bash if [ "$TERM_PROGRAM" != "vscode" ]; then # start zellij or other tooling fi ```