fix(ci): don't let a transient GitHub API error fail the commitlint gate (skyllc-ai#565)

gh api's stdout on a 5xx error is its JSON error body, but the command
still exits non-zero; the existing `|| true` swallowed the exit code
while EXISTING_ID still captured that JSON text as if it were a real
comment id. The subsequent DELETE/PATCH call then crashed on the
garbage URL, failing this *required* gate for a reason unrelated to
title conformance (release PR skyllc-ai#564 hit this: title conformed, the
stale-comment-cleanup lookup 503'd, and the whole job died).

Validate EXISTING_ID is purely numeric before trusting it; anything
else (including a JSON error blob) is treated as "no existing comment"
so the conformance result stands on its own.

Author: githubrobbi