`remote/transport.py` — build and execute `ssh`/`rsync` argv

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Overview

A transport.py module that builds and executes the ssh and rsync argv for sync, pull, and remote command execution. All external-process execution flows through one injectable runner, so tests can substitute a fake box. No ssh/rsync string is constructed anywhere else in the codebase.

Complexity: Medium
Target branch: main

Context

• Consumes the resolved profile from config.py (#703): ssh destination, remote_root, name, protect, exclude.
• The injectable runner is the seam the whole test strategy depends on — the one place a subprocess is actually spawned.

The Change

• Provide argv builders for three operations: sync, pull, and remote command execution.
• Give ssh exec two forms, because attach needs a terminal but preflight does not.
• Combine the chef's .gitignore with hardcoded protect rules so --delete cannot destroy box-managed state.
• Route every execution through one runner, so the external boundary can be replaced by a fake in tests.

Out of Scope

• tmux session semantics (session.py).
• Which command runs and how its env is assembled (driver.py).

Acceptance Criteria

• Sync argv is rsync -a --delete with --filter=':- .gitignore'.
• Sync argv carries the hardcoded protect rules plus profile protect/exclude.
• Sync transfers laptop→box only, into <remote_root>/<name>/.
• Pull transfers box→laptop, source path relative to the remote chef dir.
• ssh exec offers a non-interactive form and a -t PTY form.
• All execution flows through one injectable runner.
• No module outside transport.py calls subprocess or constructs an ssh/rsync string.

AI usage

Claude (Claude Code) drafted this from the design spec; I directed the design and reviewed it.