Fix self-test output on macOS and Linux

[corv89]

Summary

• Change test script from platform.node() to sys.version_info (avoids triggering uname -p subprocess on macOS)
• Fix output parsing to get first script line, not last summary line
• Only show output on failure, not success (cleaner status display)

Problem

Running shannot status showed jarring output:

• macOS: Created pending session for uname -p, showed "Run 'shannot approve'..."
• Linux: Showed "*** No commands or writes were queued. ***"

After

Runtime:
  ✓ Stdlib: ~/.local/share/shannot/runtime
  ✓ Sandbox binary: ~/.local/share/shannot/runtime/pypy3-c
  ✓ Self-test: passed (78ms)

[github-actions]

❌ CHANGELOG.md is out of date

Please update the changelog by running:

make changelog

Or manually:

git-cliff --config cliff.toml -o CHANGELOG.md

Then commit the updated CHANGELOG.md file.