fix: Linux CI sandbox prereqs, platform-aware tests, broader prompt stripping

- Add bubblewrap and socat to Linux CI apt-get install
- Make sandbox test assertions platform-aware (macFileSystem vs linuxFileSystem)
- Make /etc/shells test accept both macOS and Linux first-line format
- Broaden wrapped prompt fragment regex to handle path chars (ts/testWorkspace$)
- Fix continuation pattern to match user@host:path wrapped lines
- Apply stripCommandEchoAndPrompt to getOutput() in BasicExecuteStrategy
  (basic shell integration lacks reliable 133;C markers so getOutput()
  can include command echo)
- Keep RichExecuteStrategy getOutput() unstripped (rich integration
  has reliable markers)