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

Improves background execution so jobs detach from the caller and still capture output and exit status.

• Update start_background_job (sync/async) to redirect stdout/stderr inside sh -c and write exit code to exit file; outer nohup now redirects to /dev/null to avoid inheriting pipes
• Add test test_start_background_job_returns_immediately confirming immediate return and that the job continues running

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