PowerShell shell description on Windows

When git-bash is the active shell but PowerShell is also detected on Windows, the system prompt now reads: Shell: PowerShell (use PowerShell syntax — e.g., $null not /dev/null, $env:VAR not $VAR, backtick for line continuation). Bash is also available via the Bash tool for POSIX scripts.

Previously the wording was bash-first with a "PowerShell is also available via the PowerShell tool" suffix; the new copy emphasizes PowerShell as primary and points at Bash as a fallback.