Codex, one turn per post: each post in its thread from a user it hears starts
`codex exec` with the run's line and the post on its standard input; it reads
what was said before with `ocx2 thread`, and answers with `ocx2 post`. create
adds the node, process.user and the objective.

```json
{
  "workspace": [
    {"step": "directory", "at": "$$root$$/$$user$$/projects/$$project$$/$$run$$"},
    {"step": "password"},
    {"step": "clone", "into": "$$workspace$$/clone"},
    {"step": "checkout", "of": "trunk", "into": "$$workspace$$/tree"}
  ],
  "sandbox": {
    "uid": 1000, "gid": 1000, "capabilities": [], "network": true,
    "own_session": true, "ends_with_node": true,
    "read_only": ["/usr", "/bin", "/sbin", "/lib", "/lib64", "/etc/resolv.conf", "/etc/hosts",
                  "/etc/nsswitch.conf", "/etc/ssl/certs", "$$release$$"],
    "read_write": ["$$workspace$$"],
    "homes": ["/root/.codex"],
    "files": [{"at": "/etc/passwd", "text": "$$user$$:x:1000:1000::$$workspace$$:/bin/sh\n"}],
    "proc": "/proc", "dev": "/dev", "tmp": "/tmp"
  },
  "limits": {"wall": "8h"},
  "process": {
    "command": ["/usr/local/bin/codex", "exec", "--dangerously-bypass-approvals-and-sandbox", "--skip-git-repo-check",
                "Your standard input holds a line {\"run\", \"objective\"}: work toward that objective in this checkout. When a second line {\"post\", \"from\", \"text\"} follows, it is a forum post to you: read what was said before it with `ocx2 thread POST`, then answer it with `ocx2 post POST TEXT`, POST being its hash, from this directory."],
    "cwd": "$$tree$$",
    "session": true, "hears": ["stone"], "deliveries": "start", "keep": {"stderr": "1K"},
    "env": {"HOME": "$$workspace$$", "USER": "$$user$$", "PATH": "$$release$$:/usr/local/bin:/usr/bin:/bin"}
  }
}
```