Fix Claude Code CLI abnormal exit (code 159): classify fatal-signal crashes in harness retry logic

[Copilot]

Avenger's Claude Code CLI step crashed with exit code 159 (128+31 = SIGSYS) twice in one day. The harness only special-cased SIGKILL(137)/SIGTERM(143) as non---continue-able termination signals; any other fatal-signal crash (SIGSYS, SIGSEGV, etc.) fell through to the generic "partial execution" retry path, which resumes the exact on-disk session via --continue — risking an immediate repeat of whatever crashed it.

Changes

• New crash-signal classification in actions/setup/js/claude_harness.cjs: added a CRASH_SIGNAL_EXIT_CODES map for known fatal-signal exit codes (SIGILL=132, SIGABRT=134, SIGBUS=135, SIGFPE=136, SIGSEGV=139, SIGSYS=159), with isCrashSignalExitCode() and crashSignalNameForExitCode() helpers.
• Retry decision update: shouldRetryWithContinue() now treats these crash codes like the existing SIGKILL/SIGTERM handling — --continue is disabled and the next attempt starts a fresh session instead of resuming corrupted state.
• Improved diagnostics: retry logs now surface the resolved signal name (e.g. crashSignal=SIGSYS) alongside the exit code, giving future occurrences a clearer trail without requiring a core dump.
• Tests: added coverage for the new helpers and the updated shouldRetryWithContinue behavior on crash exit codes (134/139/159).

// Before: exit 159 fell into the generic "partial execution" path → retried with --continue
// After: recognized as a fatal-signal crash → --continue disabled, fresh-run retry
isCrashSignalExitCode(159); // true (SIGSYS)
crashSignalNameForExitCode(159); // "SIGSYS"

• Fixes [aw-failures] [P0] Avenger: Claude Code CLI abnormal exit (code 159) — 2nd crash today, distinct from tracked exit-139 #52727

[github-actions]

PR Triage

Category: bug | Risk: medium | Score: 66/100

• Impact: 28/50 (fixes harness mis-classification of fatal-signal crashes, improves retry reliability)
• Urgency: 20/30 (recurring crash signature, no CI signal yet)
• Quality: 18/20 (small targeted diff, clear root cause, no reviews yet)

Recommended action: fast_track
Targeted reliability fix for harness crash classification. Batched with reliability-fixes cluster (#52757, #52836, #52854).

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