Environment info now covers the GitHub CLI and its stand-in

What

Claude Code gives the model a short description of the environment it runs in, such as the platform, shell and operating system. That description now has room for the GitHub command-line tool, gh.

• githubCli is a new optional field in the environment description and in the session-environment schema, next to scratchpadDirectory.
• When gh was announced to the model earlier and is missing from a later snapshot, the model is now told that it changed. This is tracked through toldOfBuiltinGh.
• A new ghStandin slot holds a generated gh stand-in script. It can now be given --any-host when ghesHosts is "any". It then talks to github.com or to the GitHub Enterprise host a command names. Before, it accepted only an explicit --hosts= list.
• Separately, the check for placeholders that count as pasted text now also matches [ followed by a new marker.

Why

In practice the model can now be told whether gh is available and be told again if it disappears, instead of assuming a tool that is gone. Sessions that rely on the stand-in are no longer limited to a fixed list of GitHub hosts.