Investigate unclassified direct-submit first-SIGINT subprocess timeout #1069

During #1062 implementation, the real first-SIGINT subprocess check timed out while stdin remained open. Keep this symptom open until its cause is established; a later passing run does not prove a fix.

Command:

deno task test packages/cli/tests/repl-headless-submit-process.test.ts

Failing case: G: joins a blocked stdin read on the first SIGINT while its writer stays open.

First failure (local feedback, 2026-11-02 02:51 UTC; no Actions run):

G: joins a blocked stdin read on the first SIGINT while its writer stays open ... FAILED (24s)
G: retains the socket through exact Stop and held cleanup, then exits with status 130 ... ok (2s)
error: Error: The SIGINT subprocess did not join its owned exit.
FAILED | 0 passed (1 step) | 1 failed (1 step) (26s)
error: Test failed

The first command exited 1 after 29.4 seconds. Its complete original output and candidate identity remain retained in the delivery evidence.

The final subprocess run passed both steps (exit 0, 13.9 seconds) against the byte-identical candidate for signed feedback commit 4122b4324075314f1f3bb5481835f2de81aee405, based on b5cc85679742090229ee0be1707741d29f164349. The final witness acknowledges actual delivered input and records stage-specific diagnostics. Several source and fixture files changed between the failed and final candidates, so this is not proof of an identical-revision retry, and no timing-cause fix is claimed. The label tracks an unclassified intermittent symptom.

Runtime: Deno on macOS arm64. Host triage reports Deno 2.9.7 / V8 15.0.245.2-rusty / TypeScript 6.0.3; the original failing command did not print its runtime version.

Determine whether the timeout came from signal reception, input ownership, cleanup or the test handshake. Use the retained first failure and current deterministic stage diagnostics; do not close this issue merely because CI or another local run is green.