test(b5): treat zombie pane processes as stopped

[Florious95]

Goal: close the v0.5.69 Ubuntu B5 residual-sweep red without weakening leader/server survival.

Root cause: the integration fixture used kill -0 as a running-process predicate. On Unix kill -0 also succeeds for an exited but not-yet-reaped zombie. The historical Ubuntu failure spent the long residual-loop path and then reported only the worker PID as alive; neither the leader nor tmux server was reported dead. The fixture therefore misclassified a terminated worker as running.

Change: read ps STAT, treat Z as stopped, and include STAT in future worker failure output. The leader and tmux server use the same strict predicate, so a zombie leader/server still fails the survival contract.

Validation at head f34b1d7:

• Historical red: run 33325304045, job 99294264033, 4 passed / 1 failed; worker PID 25535 was the sole failed invariant.
• cargo test -p team-agent --test b5_leader_terminal_kill_red red_v3_residual_sweep_must_not_reap_leaders_tmux_server -- --exact --test-threads=1 --nocapture on macOS: 1 passed.
• Grok Linux direct clone, Rust/Cargo 1.95.0, same exact selector: 1 passed, ExecMainStatus=0, unit gb-f34b1d79-v0569-b5-linux-fixed-sol-a4.

No product lifecycle behavior, timeout, retry, serialization, or assertion scope changed.