prepare skips husky install in git worktrees, so every commit fails

[tomcasaburi]

Problem

The prepare script is test -d .git && husky install || true. In a linked git worktree, .git is a file rather than a directory, so husky install is skipped and .husky/_/husky.sh is never created. core.hooksPath=.husky lives in the shared repo config, so the hooks still run, and every commit then fails:

.husky/commit-msg: line 2: .husky/_/husky.sh: No such file or directory

Reproduction: git worktree add ../wt origin/master && cd ../wt && npm ci && git commit --allow-empty -m "test: x"

Plan

1. Change the guard to test -e .git. husky 8 itself only checks that .git exists (fs.existsSync('.git')), so it supports worktrees once it is invoked.
2. Add a test that runs the prepare script in a temporary linked worktree and in a normal checkout, and asserts that .husky/_/husky.sh exists in both.
3. npm run build && npm run build:test, npm run test:cli
4. Open a PR with Closes #N

Docker builds are unaffected: .git is in .dockerignore, and the Dockerfile installs with --ignore-scripts.

[tomcasaburi]

Fixed in #151 with test -e .git. The new test runs the real prepare script in a temporary normal checkout and a linked worktree. The worktree case fails on the old script and passes with the fix. npm run test:cli passes (45 files, 344 tests passed, 1 skipped).