fix(push): avoid process substitution in signal-sensitive job polling

[LucaLin233]

Evidence and candidate fix

Run 34226553908 reproduced trap EOF with exit 2 in a standalone Debian Bash 5.2.37 process-substitution control, without loading production code. Ubuntu reproduced the same error in the real worker signal suite. This identifies a reproducible trigger class, not Bash internal root cause.
Replace the only process substitution in tools/push.sh (job_is_active) with command-substitution snapshot plus quoted here-string. Preserve jobs -pr running-job semantics, exact PID matching and failure status; no signal masking or cleanup changes.

Tests

Add actual helper checks for active/missing/reaped direct child. Preserve original HUP/INT/TERM publication matrix plus three bounded HUP probes. Parser controls gate plain/command/snapshot modes, six delays each; handler marker and exit 129 required. Legacy known-failing process mode remains opt-in via HUP_PARSER_INCLUDE_LEGACY=true; its red evidence is preserved, not reported fixed. No assertion weakening in production regression suites.

Validation and boundaries

Three files only, based on main 597fce3. Local bash -n and diff --check passed; all behavior tests run in CI. New CI pending. No production execution, deployment, real credentials, xtrace, timeout increases, or #41 changes. #47 remains blocked until candidate validated and reviewed. Revert this PR for rollback. Draft; do not merge without explicit confirmation.

[LucaLin233]

Added tests/test-hup-parser.sh: 18 bounded inert controls (plain loop, command substitution, process substitution; six fixed delays each), 4s timeout plus 1s kill grace per case. Requires HUP-handler marker AND exit 129, fails on first bad result, no retry-to-green. Existing tests/test-*.sh selection covers the new suite without mapping changes; worker-registration suite remains selected by its existing diff. No production code change. This is a candidate reproducer, not confirmed reproduction or proof of root cause; marker/delay cannot fix exact parser timing. Local syntax/diff checks only; CI pending.