Follow-up: cache-diagnostics.sh swallows warm-rebuild failures instead of failing CI on them

[claude]

Follow-up to review feedback on #231 (ci: add a BuildFetch build-cache diagnostic), left unaddressed at merge time.

chatgpt-codex-connector[bot] (P2) flagged that scripts/cache-diagnostics.sh pipes the warm/cacheability rebuild through 2>&1 || true, and the script's final exit status only checks whether reloc_count == 0. A genuine failure during the warm rebuild (crash, compile error, test failure) is silently swallowed rather than failing the diagnostic job.

Still present on main today:

scripts/cache-diagnostics.sh:101:    --console=plain "${TASKS[@]}" ) > "$WORK/warm.raw" 2>&1 || true
scripts/cache-diagnostics.sh:153:[ "$reloc_count" -eq 0 ]

No reply or follow-up commit addressed this comment before #231 merged.