fix for sandbox start_background_job

[kcoopermiller]

# Takes 5 seconds (backgrounded process inherits pipes, blocks until exit)
prime sandbox run <sandbox_id> "nohup sh -c 'sleep 5 &'"

# Returns immediately (redirections prevent pipe inheritance)
prime sandbox run <sandbox_id> "nohup sh -c 'sleep 5 > /dev/null 2>&1 &'"

Note

Adjusts how background jobs capture output and exit codes for reliability.

• In start_background_job and async counterpart, redirect stdout/stderr within the inner sh -c subshell to *.stdout.log/*.stderr.log while always writing exit code to *.exit
• Change outer nohup to < /dev/null > /dev/null 2>&1 since logging is handled inside the quoted command

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