[BUG] Platform reported as "darwin" / Shell "zsh" under Git Bash (MSYS) on Windows 11

Summary

When running Claude Code inside Git Bash / MSYS on Windows 11, the environment block injected into the session reports Platform: darwin and Shell: zsh. The machine is Windows 11 with no macOS whatsoever. This causes the model to mis-judge the environment (e.g. assuming macOS paths/shell, or that D:\ drives are inaccessible), leading it to use wrong path conventions and refuse valid operations.

Environment

• OS: Windows 11 (build 26200)
• Shell: Git Bash / MSYS (MINGW64)
• Claude Code version: 2.1.170
• Node: v24.12.0

Real environment values (from the shell Claude Code itself runs in):

$ uname -a
MINGW64_NT-10.0-26200 Maxwell-OMEN 3.6.6-1cdd4371.x86_64 2026-01-15 22:20 UTC x86_64 Msys

$ echo $OSTYPE
msys

$ echo $MSYSTEM
MINGW64

Actual behavior

The session environment block contains:

Platform: darwin
Shell: zsh

These are macOS defaults. darwin + zsh appear together as a set, which strongly suggests the platform-detection logic fails to recognize the MSYS/MINGW environment and falls back to hard-coded macOS default values.

Expected behavior

On Git Bash / MSYS, Claude Code should detect the environment from uname -s (which contains MINGW64_NT / MSYS_NT) and/or $OSTYPE (msys) and $MSYSTEM (MINGW64), and report the platform as Windows (MSYS/MinGW) with the correct shell (bash), instead of falling back to darwin / zsh.

Impact

Because the injected environment block claims macOS, the model:

• assumes macOS-style paths and shell semantics;
• can wrongly conclude Windows drives (e.g. D:\) are inaccessible, when in fact they are reachable via /c/, /d/ under MSYS;
• adopts incorrect path conventions (D:\... vs /d/...).

This is closely related to #9883 (MSYS environment not recognized, cygpath handling), but the specific symptom here is the platform/shell metadata being reported as darwin/zsh, which I could not find reported elsewhere.

Suggested fix

Detect MSYS/MinGW via uname -s (MSYS_NT / MINGW64_NT / MINGW32_NT) or $OSTYPE=msys / $MSYSTEM, and populate the environment block with the correct Windows-under-MSYS platform and bash shell, instead of the macOS fallback.