Passive write_stdin polling can return exit_code=143 without a correlated inner-command receipt

Environment

• Linux (Strato2 remote compute host)
• Managed Codex unified exec; reporter version was not captured
• Same-host control: codex-cli 0.154.0
• Long command wrapped by KB_COMPACT_KEEP_LOG=1 kb-run-compact ...

Intermittent reproduction

1. Start a multi-minute foreground command through exec_command with a short yield_time_ms so it returns an inner session_id.
2. Keep the owning turn active.
3. Poll that same id with empty write_stdin calls. Do not send Ctrl-C or any termination request.
4. Intermittently, a poll returns exit_code=143 around three minutes even though the command wrapper emitted no correlated terminal receipt.

Observed on a Julia/Documenter build: the last model-visible heartbeat was [kb-run-compact +161s ...] ExpandTemplates; the retained 13,699-byte log ended at ExpandTemplates. It contained neither [kb-run-compact] exit=... nor the wrapper's retained this wrapper received SIGTERM trap stamp. Child completion/cancellation was therefore unknowable, and launching a duplicate was unsafe.

Controls

• On the same host with Codex 0.154.0, the same yielded-session/empty-poll pattern ran 221 seconds and completed with tool exit 0 plus the wrapper's final exit=0 receipt. This is not a deterministic three-minute ceiling.
• Deliberately TERM-signaling the wrapper produced all correlated evidence: tool 143, terminal wrapper exit=143, and a retained this wrapper received SIGTERM stamp. The passive incident had none of those wrapper signals.

Expected behavior

A yielded exec session should have a durable, runtime-owned status/lookup channel. Any terminal tool status must be correlated to the actual inner command, or explicitly labeled as transport/session termination. After a polling route is lost or passively settles, callers need to retrieve the inner command's final exit status or verify/cancel it without cross-session PID guessing.

Related: #11058 and #14824.

Start by tracing the exec_command and write_stdin session lifecycle, then run the yielded-session/empty-poll reproduction and compare it with deliberate SIGTERM behavior. Done means terminal status is correlated to the inner command or explicitly identified as transport termination, with a way to retrieve or verify the command's final status after polling is lost.