[core] Consolidate setup scripts into a cross-platform setup.py

script/setup and script/setup.bat implemented the same workflow and were
kept in sync by hand, which had already drifted: the worktree hook fix in
#18843 landed in the bash script only. Both are now thin wrappers around
script/setup.py, so there is one implementation to maintain.

The post-checkout hook also checks for a venv/Scripts layout before doing
anything, so a branch switch on Windows can no longer clear a working
virtual environment.

script/git-hooks/post-checkout

@@ -1,20 +1,38 @@
 #!/bin/sh
 # Prepare the dev environment for a new checkout or worktree.
 #
-# Installed into the git hooks directory by script/setup. Deliberately tiny and
-# self-contained: it stays valid on branches where script/setup does not exist,
-# and simply does nothing there.
+# Installed into the git hooks directory by script/setup.py. Deliberately tiny
+# and self-contained: it stays valid on branches where the setup script does not
+# exist, and simply does nothing there.
 
 # $3 is 1 for a branch checkout, 0 for a file checkout.
 [ "$3" = "1" ] || exit 0
 
 top=$(git rev-parse --show-toplevel 2>/dev/null) || exit 0
 
-# This also runs on ordinary branch switches, where there is nothing to do.
+# This also runs on ordinary branch switches, where there is nothing to do. Both
+# layouts are checked because git for Windows runs hooks under its own bundled
+# shell, where the environment lives in venv/Scripts rather than venv/bin.
 [ -x "$top/venv/bin/python" ] && exit 0
-[ -x "$top/script/setup" ] || exit 0
+[ -f "$top/venv/Scripts/python.exe" ] && exit 0
+
+[ -f "$top/script/setup.py" ] || exit 0
 
 # Clear VIRTUAL_ENV so a checkout made from a shell with an environment already
 # activated still gets its own, rather than having the active one repointed at
 # this working tree.
-exec env -u VIRTUAL_ENV "$top/script/setup"
+unset VIRTUAL_ENV
+
+# The interpreter goes by different names across platforms, and on Windows
+# "python3" is often a stub that opens the app store instead of running
+# anything, so each candidate is tried before it is used. Doing nothing is the
+# right outcome when none of them work.
+try_setup() {
+  "$@" -c "" >/dev/null 2>&1 || return 1
+  exec "$@" "$top/script/setup.py"
+}
+
+try_setup python3
+try_setup python
+try_setup py -3
+exit 0