Self-review of the previous commit. Folding the head and base re-reads into one
`prjson_now=$(api_get ... || true)` swallowed a guard that used to be implicit: the old
`sha_now=$(api_get ... | jq ...)` aborted under `set -e` + `pipefail` when the GET
failed, before any status was written. With `|| true`, both `sha_now` and `base_now`
come back empty, both `[ -n ... ]` guards no-op, and the status is written having
confirmed nothing about either the head or the base — a fail-open regression introduced
by the refactor itself.
Confirmed the old behaviour empirically rather than by reading it: a failed piped command
substitution under `set -euo pipefail` exits with curl's status.
The refusal is now explicit, and pinned by a test — nothing asserted it before, which is
exactly why the refactor could drop it silently. Mutation-verified: restoring `|| true`
reddens that test alone.
Refs #632
timothy
f0f8708a6e