Pull request security flaw found: shell string injection risk in agent tool

A Friday incident temporarily halted a small checkout service due to a failing job from an agent's pull request. The submitted patch introduced a helper function that constructed a shell command using an uncontrolled, user-supplied branch name from tool JSON, creating a security vulnerability. The proposed fix rejects unsafe shell strings by validating the branch name with a strict pattern and using execFile with separate argument arrays instead. It also adds automated tests to ensure the handler does not use shell mode and that invalid branch names are rejected before any processes start.