fix(tests): make worktree/sanity tests pass on ubuntu CI

Two independent, pre-existing test bugs surfaced on ubuntu-latest (macos
passed) after a GitHub runner image change plus the find-based test order:

1. create_test_repo ran git init + git commit in a sandboxed empty $HOME
   without configuring user.name/user.email, relying on the runner's
   ambient git config. The current ubuntu image no longer provides one, so
   the commit aborted with 'fatal: empty ident name not allowed'. Set an
   explicit local git identity in the sandbox repo.

2. The runner sets the sandbox $HOME once and relies on each test file's
   cleanup to leave it existing for the next file. sanity's cleanup did
   'rm -rf $HOME' without recreating it, unlike worktree's cleanup. find
   orders sanity before worktree on ubuntu, so worktree's setup
   'touch $HOME/$fingerprint' silently failed and the teardown's
   assert_file_exists then failed, making the file exit non-zero even
   though all assertions passed. Add 'mkdir -p $HOME' to match worktree's
   cleanup convention. This was masked until bug #1 was fixed, because
   finish_tests exited before cleanup ran while tests were failing.

Author: sha1n

tests/sanity.test.sh

@@ -24,6 +24,7 @@ cleanup() {
   if [[ -f "$HOME/$fingerprint" ]]; then
     echo
     rm -rf "$HOME"
+    mkdir -p "$HOME"
   fi
 }
 

tests/worktree.test.sh

@@ -31,6 +31,8 @@ create_test_repo() {
   mkdir -p "$repo_dir"
   cd "$repo_dir"
   git init -q
+  git config user.email "test@example.com"
+  git config user.name "Test"
   git commit -q --allow-empty -m "initial commit"
 }