gh_issue_close fails: unknown JSON field closedByPullRequestsUrls

[four-bytes-robby]

Context

gh_issue_close crashes when viewing the target issue:

Error viewing issue #N: gh exited with code 1: Unknown JSON field: "closedByPullRequestsUrls"

The gh CLI renamed this field; the current valid field is
closedByPullRequestsReferences (an array of objects, each with number,
title, url).

Scope

src/tools/gh-issue-close.ts:

• line 66: change the --json field list from closedByPullRequestsUrls to
  closedByPullRequestsReferences
• update the IssueView interface (line 16) and the zombie-detection loop
  (lines 81-94) to match the new object shape (iterate {number, url} refs,
  render url (PR #number))

Definition of Done

• bun run lint:ts (tsc --noEmit) clean
• bun test passes
• bun run build regenerates dist/four-opencode-git.js (committed)
• Zombie detection still works (array-empty => not zombie)