task install-githooks overwrites existing pre-push hook

The `task install-githooks` command copies `githooks/pre-push` to `.git/hooks/pre-push`, overwriting any existing hook without warning.

Users with custom pre-push hooks lose their configuration.

Start with the `task install-githooks` entry and inspect how it copies `githooks/pre-push` to `.git/hooks/pre-push`. Check the existing-hook behavior, then verify that a custom pre-push hook is not silently overwritten and that users receive an appropriate warning.

Written by the indexing model from the issue text.