fix: detect the active Windows shell

[huyua9]

Problem

On Windows, ai-shell always reports PowerShell to the model even when it is launched from cmd, Git Bash, or an MSYS terminal. That can produce commands for the wrong shell.

What changed

• Prefer SHELL when it identifies the active shell, including Windows paths and .exe names.
• Recognize Git Bash/MSYS through MSYSTEM when SHELL is unavailable.
• Fall back to ComSpec/COMSPEC and distinguish cmd from PowerShell.
• Add regression checks for Git Bash, MSYS, cmd, and PowerShell cases.

Validation

• ./node_modules/.bin/jiti tests/os-detect.test.ts
• npm run typecheck
• npx prettier --check src/helpers/os-detect.ts tests/os-detect.test.ts
• npx eslint src/helpers/os-detect.ts tests/os-detect.test.ts
• git diff --check