fix(windows): correct shell flag for Git Bash / POSIX shells (fix #7)

[sinouw]

Closes #7.

Full reproduction and environment details: #7.

Problem

On Windows, lean-ctx -c <command> failed with output like /C: Is a directory and exit code 126 in Git Bash and when SHELL points at a POSIX shell (e.g. bash.exe). Global shell hooks (alias git='lean-ctx -c git', etc.) were unusable in those environments.

Root cause

detect_shell() returns SHELL when set (e.g. Git for Windows sets it to bash.exe). shell_and_flag() treated every non–PowerShell executable on Windows as cmd.exe, passing /C. That switch is only valid for cmd.exe; bash expects -c. Invoking bash with /C leads to the MSYS-style error and 126.

Solution

Shell executable (basename)	Flag
powershell / pwsh	-Command
cmd.exe / cmd	/C
bash, sh, zsh, fish, … (default)	-c

Implemented in windows_shell_flag_for_exe_basename() and wired through shell_and_flag() so exec, -c passthrough, and ctx_shell-style spawn paths stay consistent.

Testing

• Unit tests for windows_shell_flag_for_exe_basename (cmd, PowerShell, bash, etc.) in rust/src/shell.rs
• cargo test: all tests passed in Docker rust:1.88-bookworm (contributor host lacked MSVC link.exe; CI should confirm)
• Manual (Windows + Git Bash): lean-ctx -c "git --version" and lean-ctx -c "echo ok" should succeed after build

Checklist

• Targeted change (shell.rs only); no unrelated refactors
• Linux/macOS: behavior unchanged (-c only branch)

[yvgude]

Tested root cause analysis: detect_shell() returns SHELL env var (bash.exe on Git for Windows), but shell_and_flag() treated all non-PowerShell Windows shells as cmd.exe and passed /C. Fix is correct — proper detection of POSIX shells (bash/sh/zsh/fish) and assignment of -c flag. Unit tests are solid. Merging.