Add GitLab access token authentication for git push

When the environment variable GITLAB_TOKEN (or GITLAB_ACCESS_TOKEN) is set, git commands that contact a remote (push, fetch, pull, clone, ls-remote) automatically use the token for HTTPS authentication.

Implementation:

• New module klausagent/tools/git_credentials.py: builds a credential helper via GIT_CONFIG_COUNT env vars so no token touches the disk and no remote URLs are modified.
• git_tools_server.py: git_push and repo_run (for remote git commands) now call _remote_env() to inject credentials when available.
• tests/test_git_credentials.py: 12 unit tests covering token lookup, env construction, precedence, and env inheritance.
• README.md: documents the GITLAB_TOKEN environment variable.