fix: isolate smoke test from user's real config directory

[Copilot]

The smoke test was deleting ~/.config/c8ctl/session.json directly, risking disruption of a developer's local environment when run manually.

Changes

• At script startup, create a temp directory and export it as XDG_CONFIG_HOME, redirecting all CLI config/session I/O away from the user's real ~/.config/
• Register a trap to delete the temp dir on EXIT (success or failure)
• No changes needed to cleanup_session — it already resolves config dir via ${XDG_CONFIG_HOME:-$HOME/.config}, so it transparently targets the isolated dir

TEST_TMPDIR=$(mktemp -d)
export XDG_CONFIG_HOME="$TEST_TMPDIR"
trap 'rm -rf "$TEST_TMPDIR"' EXIT

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