add shell command support to InOut

[perNyfelt]

What changed

• Add sh() for concise shell execution returning captured stdout.
• Add shell() and ShellResult for stdout, stderr, exit-code, and success handling.
• Execute commands through the platform shell with concurrent output draining and live streaming.
• Add coverage for exit status, stderr, streaming, globbing, pipelines, and redirects.
• Document usage, platform behavior, and shell-injection precautions.

Validation

• ./gradlew test
• git diff --check

The PR is ready for review.