Fix gh api stdout-on-error bug in close-linked-issues.yml #370

The parent-issue lookup step in .github/workflows/close-linked-issues.yml
(added in #369) breaks on any issue with no parent (e.g. a standalone
scripts/flow.py issue ticket, or an OpenSpec change's own parent issue).

`gh api ... --jq '.number'` writes the raw error JSON body to STDOUT
(not just a message to stderr) when the HTTP call itself fails, then
exits 1. The `parent=$(... 2>/dev/null || echo "")` fallback only
discards stdout on failure via the OR branch, but by then the error
body is already captured inside the command substitution, concatenated
with echo's empty output — so $parent ends up holding the raw error
JSON, which then gets used as a URL path segment in the next gh api
call and fails with "unsupported protocol scheme".

Confirmed live: PR #369's own merge run closed #368 successfully, then
crashed exactly this way since #368 (a standalone ticket) has no parent.

Fix: check the command's own exit status (`if ! parent=$(...); then`)
instead of trusting the captured string, so a failed lookup is detected
even though gh still wrote something to stdout. Verified locally with a
mock gh reproducing the 404-writes-to-stdout behavior, and against the
live #363/#361 parent/summary lookup for the success path.