PRD 0021 chunk 1. The tmux split-pane helpers (chunk 2+) need
the same docker-exec argv that `exec_claude` builds — including
the `--append-system-prompt-file <path>` flag the bottle's
provisioner copies into place. Extract the argv construction
into a pure `claude_docker_argv(argv, *, tty)` method so both
foreground (`subprocess.run`) and tmux paths
(`tmux respawn-pane …`) build from the same source.
`exec_claude` becomes a one-liner that runs subprocess.run on
the argv. No behavior change; 472 unit tests pass (7 new for
the pure builder).
didericis
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