CVE-2026-55158: Conflibot: Command injection via crafted pull request branch names under pull_request_target

Conflibot warns in advance when merging a pull request will cause conflicts in other open pull requests. Prior to 1.2.1, src/index.ts builds git checkout, git merge, and git format-patch commands by interpolating the attacker-controlled pull request head.ref value into strings passed to exec. In the documented pull_request_target configuration, an attacker can open a pull request, including from a fork, whose branch name contains shell metacharacters, and the workflow automatically interprets those characters as commands without maintainer interaction. The commands execute on a runner with base-repository secrets and a write-scoped GITHUB_TOKEN, allowing arbitrary command execution, secret or token exfiltration, unauthorized pushes, and other token abuse. The fixed implementations in src/index.ts and src/conflibot.ts use execFile or spawn argument arrays, and the v2 line also uses numeric pull-request refs rather than branch names. This issue is fixed in versions 1.2.1 and 2.0.0.