fix(sdk): nohup mode always returns exit_code=0 regardless of command result

[Issac-Newton]

Problem

Sandbox.arun() in nohup mode always returns exit_code=0 when the process completes within wait_timeout, regardless of whether the command actually succeeded or failed.

result = await sandbox.arun(cmd="nonexistent_command_xyz", session="s", mode="nohup")
# Current: result.exit_code == 0  ← wrong, command not found should be 127

This makes it impossible for callers to distinguish between a successful command and a failed one via exit_code.

Root Cause

handle_nohup_output hardcodes exit_code=0 when success=True (process completed within timeout), with no mechanism to capture the actual exit status of the background command.

Proposed Fix

Add a capture_exit_code: bool = False parameter to arun(). When True, the nohup command is wrapped in a subshell that writes the exit code to a .rc file:

# before
nohup {cmd} < /dev/null > {out} 2>&1 & echo PIDSTART${!}PIDEND;disown

# after (capture_exit_code=True)
( nohup {cmd} < /dev/null > {out} 2>&1; echo $? > {rc} ) & echo PIDSTART${!}PIDEND;disown

After the process completes, the .rc file is read to obtain the actual exit code.

The flag defaults to False for full backward compatibility.

Acceptance Criteria

• arun(cmd="nonexistent_command", mode="nohup", capture_exit_code=True) → exit_code=127
• arun(cmd="exit 42", mode="nohup", capture_exit_code=True) → exit_code=42
• arun(cmd="echo hello", mode="nohup", capture_exit_code=True) → exit_code=0
• Default capture_exit_code=False behavior unchanged
• Timeout still returns exit_code=1