flow next: find a PR pushed under a different public branch name

[TomasPalsson]

What breaks

In stealth mode /flow:next ships with git push -u <remote> HEAD:<plain-name> and gh pr create --head <owner>:<plain-name>, often from a fork. The router's prNumber only runs gh pr view <local-branch>, so a PR opened from local flow/office-previews under the public head TomasPalsson:feat/storage-backed-office-previews is never found, and flow next stays in shippable ("no PR … opening it") after the PR exists.

After this change

prNumber still tries gh pr view <local-branch> first. When that finds nothing, it reads the branch's upstream (branch.<b>.merge, branch.<b>.remote), takes the owner from that remote's URL (git@host:Owner/Repo, https://host/Owner/Repo, ssh://git@host/Owner/Repo), and runs gh pr list --head <upstream-name> --state all, keeping only a PR whose head repository belongs to that owner. No upstream, no parseable owner, or any gh failure → no PR, as before.

--author @me was deliberately not used: in a transferred repository (LibreChat-AI/LibreChat) it returned no PRs at all, including the user's own.

Tests

• TEST_ONLY=test_next.sh bash plugins/flow/scripts/tests/run.sh → 182 passed, 0 failed (178 before; new test failed first).
• New test: no upstream → shippable; upstream with an unparseable remote URL → shippable; upstream on git@github.com:someone/repo.git with a stubbed gh → shipped, PR #77.
• Real check: on the branch above, flow next now reports shipped with the opened PR's number. The https:// and ssh:// URL forms are covered by reading the regex only.