fix: keep terminal helper script alive until child exits

[alexsmolya]

terminal-helper currently deletes the generated temporary script after the terminal launcher returns. However, kgx --wait does not guarantee that the child has already consumed the script, so the command can fail before pkexec/bash opens it.

This change:

• moves normal cleanup ownership into the generated script with an EXIT trap that preserves the child exit status;
• keeps launch-failure cleanup parent-owned;
• leaves terminal selection, the pkexec flow, and package/kernel command construction unchanged;
• adds a deterministic regression test covering delayed child startup, the rootshell.sh/pkexec path, non-zero child exit, and terminal launch failure.

Validation:

• bash -n src/terminal-helper src/rootshell.sh tests/terminal-helper_test.sh
• tests/terminal-helper_test.sh
• git diff --check

The tests use harmless marker commands only; no real package, kernel, or root operation is needed.

Fixes #87