Add a managed .NET bootstrap path for the sandbox tooling baseline

[eigenverft]

Summary

Add a managed .NET bootstrap path so the sandbox has the baseline local tooling needed for Codex-assisted build and repo tasks.

Source

Tracked from source/PROJECT-TODO.md under [P3] Normal / Tooling.

Still Open

The current repo surface and README.md do not yet expose a managed .NET bootstrap command or documented .NET bootstrap path.

[eigenverft]

Closing: covered by the successor project Eigenverft.Manifested.Sandbox.