Closes #275

Summary

Fixes shell auto-detection in testsprite completion when invoked without an explicit shell argument.

Problem

Currently, detectShell(env) in src/commands/completion.ts uses shellPath.slice(shellPath.lastIndexOf('/') + 1).
On Windows or cross-platform environments using Git Bash / MSYS2 / WSL:

1. $SHELL uses backslashes (e.g. C:\Program Files\Git\bin\bash.exe). lastIndexOf('/') returns -1, resulting in the full path string C:\Program Files\Git\bin\bash.exe which fails isShell().
2. $SHELL filenames end in .exe (e.g. bash.exe, zsh.exe, fish.exe), causing isShell("bash.exe") to return false.

Solution

Enhances detectShell(env) to:

• Split on both forward / and backward \ slashes (/[/\\]/).
• Lowercase and strip trailing .exe extensions (.toLowerCase().replace(/\.exe$/, '')).
• Match against isShell() (bash, zsh, fish).

Test Coverage

Updated src/commands/completion.test.ts with unit tests for Windows paths, .exe suffixes, mixed slashes, and uppercase names. All 2,002 tests pass.