dsh-fleet-audit: A read-only credential hygiene audit for DSH agent machines

## Introduction In the era of multi-agent development environments, credentials are scattered across `~/.gitconfig`, agent configurations, `.git/config`, and various `.env` files. If Git's `url.*.insteadof` or `pushurl` settings embed a token (e.g., `https://oauth2:<token>@gitlab.example.com/...`), every invocation of `git remote -v` will expose the secret in logs, chat sessions, or CI pipelines. The traditional approach involves manually checking file permissions and grepping remote configurations one by one, which is time-consuming and error-prone. dsh-fleet-audit automates this process...