Convert POSIX script paths to Windows form before exec on Git Bash

Fixes #2043. On Git Bash for Windows, Claude Code hands script paths to
the shim in POSIX form (`/c/Users/...`). We exec a Windows `python.exe`
(the `python3` Microsoft Store stub fails the probe), and Windows Python
interprets the leading `/` as the root of the current drive — `/c/...`
becomes `C:\c\Users\...` or `D:\c\Users\...` depending on which
drive the shell happens to be on, fails with ENOENT, and every
Edit/Write/MultiEdit blocks until the session restarts.

Convert absolute path args via `cygpath -w` (a Git Bash builtin) before
exec. Guarded by `command -v cygpath` so macOS/Linux fall straight
through unchanged; `cygpath -w` is idempotent on already-Windows paths
so the rare mixed-form case is safe. Only `/*` paths are converted —
Windows-form paths reaching the shim are already openable by python.exe.

Verified locally:
- cygpath absent on macOS → guard skips → POSIX behavior unchanged
- end-to-end shim invocation with a POSIX path on macOS exits 0
- stubbed cygpath -w on /c/Users/test/hook.py produces C:\Users\test\hook.py

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugins/security-guidance/hooks/sg-python.sh

@@ -22,6 +22,29 @@
 #        "${CLAUDE_PLUGIN_ROOT}/hooks/security_reminder_hook.py"
 set -e
 
+# Git Bash / MSYS on Windows hands script paths to this shim in POSIX form
+# (`/c/Users/...`). When we exec a Windows `python.exe` (which we do on
+# Windows since `python3` is the Microsoft Store stub), python interprets the
+# leading `/` as the root of the current drive — e.g. `/c/Users/...` becomes
+# `C:\c\Users\...` or `D:\c\Users\...` (whichever drive the shell is on),
+# fails with ENOENT, and every Edit/Write/MultiEdit tool use blocks until the
+# session restarts. See anthropics/claude-plugins-official#2043.
+#
+# Fix: convert absolute path args to native Windows form via `cygpath -w`
+# before exec. `cygpath` is a Git Bash builtin; it's absent on macOS/Linux,
+# where the `command -v` guard makes this a no-op. `cygpath -w` is idempotent
+# for already-Windows paths so the rare mixed-form case is safe.
+if command -v cygpath >/dev/null 2>&1; then
+    converted=()
+    for a in "$@"; do
+        case "$a" in
+            /*) converted+=("$(cygpath -w "$a")") ;;
+            *)  converted+=("$a") ;;
+        esac
+    done
+    set -- "${converted[@]}"
+fi
+
 probe() {
     # $1..N: the interpreter command (may be multi-word like `py -3`)
     # Probe writes the major version to stdout and exits 0 iff it's >=3.