fix: resolve packaged zstack directory in debug script

[tllovesxs]

Summary

• Only select packaged zstack directories when locating the debug runtime.
• Exit with the existing friendly mvn package message when no distribution exists.
• Make the script's relevant path handling and test expressions POSIX-shell compatible.

Closes #12.

Test plan

• sh -n build/debug.sh
• Run build/debug.sh without a packaged distribution and verify it exits with status 1 and prints the existing mvn package guidance.

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