hooks: leave pre-commit's migration mode in place; it runs Entire's hook

pre-commit install after entire enable moves Entire's hook to <hook>.legacy and runs it from pre-commit's wrapper, failing the hook when it fails. Entire's hook still runs. The previous branch keyed only on the Entire-marked .legacy and moved whatever hook was current aside: for pre-commit's wrapper that silenced pre-commit's checks for the hook, and the next pre-commit install plus an agent turn repeated it forever; for any other hook, a stale .legacy disabled the newest hook.

Recognise the arrangement only when the hook is pre-commit's wrapper (its header or one of pre-commit's own script IDs, matched anywhere for the Windows shebang) and .legacy carries Entire's marker. That counts as installed for the hook-state check, and install refreshes Entire's hook in .legacy instead of replacing the wrapper. Any other hook takes the normal backup path whatever .legacy holds.