Add CLI sub-agent workflows

[Aaronontheweb]

Closes #91

Summary

• add lint subagent validation for NetClaw-compatible agent markdown files
• add publish-subagent with required packaging version, duplicate skip, force, dry-run, and verbose output
• add download-subagent as a destination-agnostic verified artifact sync primitive
• document sub-agent CLI workflows and adapter-owned destinations

Tests

• dotnet build -c Release
• dotnet test -c Release
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• dotnet slopwatch analyze
• git diff --check