fix(installer): guide Windows shell users to PowerShell (#1294) (#2048)

Recognize MINGW, MSYS, and Cygwin and print the exact PowerShell installer command.
Keep Darwin/Linux behavior unchanged and exit non-zero before downloading.
Cover shell dispatch and real Windows ARM64 Git Bash behavior.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

install.sh

@@ -34,6 +34,11 @@ arch="$(uname -m)"
 case "$os" in
   Darwin) os="darwin" ;;
   Linux)  os="linux" ;;
+  MINGW*|MSYS*|CYGWIN*)
+    echo "codegraph: on Windows, open PowerShell and run:" >&2
+    echo "  irm https://raw.githubusercontent.com/colbymchenry/codegraph/main/install.ps1 | iex" >&2
+    exit 1
+    ;;
   *) echo "codegraph: unsupported OS '$os'." >&2; exit 1 ;;
 esac
 case "$arch" in