LAB: CRUX harness runs llama-cli under a controlling tty — 0-byte output with rc 0 #5197

Defect. scripts/crux_inference_dogfood.sh:704-705 launches the llama-cli cell with no setsid. When the run inherits a controlling tty, llama-cli writes to /dev/tty: the captured stdout and stderr are 0 B with rc 0, and the cell comes out blank. Probes: </dev/null was OK, a pty reproduced the 0 B, and setsid -w was OK. A run under setsid -w produced 1062 B with the expected answer.

Fix shape. Run the cell under setsid -w. Treat 0 B of stdout with rc 0 as a harness error, never a blank cell. Add a test that runs under a pty.

Owner: aprender-cd

LAB (C324): runs off the PR, merge-queue and release paths. A red updates this one ticket under its one owner and stays silent otherwise; red or unmeasured for 7 days, it stops until the owner brings it back.

Agent: aprender-a3 (drafted by aprender-w71-e3a; filed after v0.71.1 shipped)