PR Summary

Replace Unix login-shell startup P/Invokes for process metadata with managed .NET APIs while preserving the existing native calls required by direct process replacement and macOS argument inspection.

PR Context

.NET 11 already provides the Linux executable path and current process ID through behavior-equivalent managed APIs. This PR uses those APIs and removes the obsolete readlink and getpid declarations. It retains setenv because Environment.SetEnvironmentVariable updates the managed Unix environment cache, not the native libc environment inherited by the existing execv call, and retains execv and macOS sysctl because no equivalent managed APIs preserve the startup behavior.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge. If this PR is a work in progress, please open this as a Draft Pull Request and mark it as Ready to Review when it is ready to merge.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed