Add Windows Command Line Best Practices to AGENTS.md

[NachoEchevarria]

Summary of changes

Added clear guidelines to AGENTS.md about avoiding >nul and 2>nul redirections on Windows, which can create literal "nul" files that are extremely difficult to delete.

Reason for change

On Windows, redirecting to nul (e.g., command 2>nul) can create a literal file named "nul" in the current directory instead of properly redirecting to the NUL device. These files are problematic because:

• They cannot be deleted using standard Windows commands
• They cause repository hygiene issues
• They're confusing and frustrating to deal with

This issue was documented in anthropics/claude-code#4928

Implementation details

1. Added new section: "Windows Command Line Best Practices" in AGENTS.md after the Coding Standards section
2. Provided clear examples of problematic commands (including findstr with 2>nul)
3. Documented safe alternatives:
   • Let error output show naturally (don't suppress)
   • Use full device path \\.\NUL if suppression is essential
   • Prefer PowerShell or dedicated tools over piped bash commands

Test coverage

Other details

[bouwkast]

FWIW whenever this happened I just started a WSL instance in the terminal and ran git clean -fd