Clean room: name the in-box C# toolchain in ENVIRONMENT.md

[allens]

The Windows restore brief told the session to use "the most modern version of the language installed on this machine". ADR-0096 says the opposite: C# on the .NET Framework that ships with Windows, nothing installed. The first Windows restore experiment (2026-10-16, Sonnet 5.5, medium) followed the brief: it searched the machine for a compiler, settled on Visual Studio's Roslyn csc and the .NET 10 runtime, and one of its searches, Get-ChildItem $env:USERPROFILE\.dotnet, was a look into the home folder that voided the run.

Three changes, all on the Windows restore build:

• The brief's sentence now names the in-box framework (cleanroom.mjs).
• The build finds the framework's csc.exe and Windows PowerShell at build time and names them in a Toolchain section of ENVIRONMENT.md (csharpToolchain), telling the session to use those and not to look for another. Found at build time rather than named in prose, where a version would rot; a Windows with no .NET Framework 4 fails the build. On this machine it names C:\WINDOWS\Microsoft.NET\Framework64\v4.0.30319\csc.exe (C# 5) and powershell.exe for Add-Type.
• The README's Windows paragraph, which said "install it before the run", and ADR-0096's Windows bullet are rewritten in place to match.

Checked by writing a restore clean room with fake credentials and reading its ENVIRONMENT.md and brief; no build or run.

🤖 Generated with Claude Code

[allens-claude-reviewer]

Code review (Opus, high): 7 findings. The first three compare.py findings were confirmed by running it on Windows (Python 3.14.5): each prints IDENTICAL and exits 0.

scripts/cleanroom/build-restore-cleanroom.mjs:179 — The build's handover still says to install a toolchain: "install the toolchain: nothing comes as standard on Windows", the claim this PR reverses in the brief, README and ADR. An operator following it installs the .NET SDK or Visual Studio while the brief the same build just wrote tells the session to use the in-box framework and nothing newer. The README's twin of this sentence was removed; this one was not.

scripts/cleanroom/README.md:162 — This section still says the brief "names no version and no toolchain", leaving the session to find "the most modern version that comes as standard". On Windows the brief now names the .NET Framework and ENVIRONMENT.md names a csc.exe path, so line 162 and the Windows paragraph at line 239 contradict each other within one file. The module comment at cleanroom.mjs:28-31 says the same thing.

scripts/cleanroom/cleanroom.mjs:424 — The ground rule still says ENVIRONMENT.md says only "where the bucket is", while the brief three paragraphs earlier sends the session there for the compiler path. The two instructions about the same file disagree.

[allens]

On the three findings in the review body, all fixed in 820ac66:

• build-restore-cleanroom.mjs:179 (the handover still says to install a toolchain): the item is gone; handover now prints its "Still to do" list only when there is one (cleanroom.mjs). The build itself refuses when the compiler is missing, so there is nothing left to tell the operator.
• README.md:162 (the brief "names no version and no toolchain"): now "names no version … Windows is the exception, below", in the README and the module comment both.
• cleanroom.mjs:424 (the ground rule says ENVIRONMENT.md says only where the bucket is): on Windows it now says "where the bucket and the compiler are".