fix(v1.27 migration): qualify repo with owner so gh rename actually runs

[agile-operators]

Summary

The v1.27.0.0 migration (gstack-brain → gstack-artifacts rename) silently fails to rename users' GitHub repos because it passes a bare repo name to gh repo rename --repo and gh repo edit, both of which require the [HOST/]OWNER/REPO form.

$ gh repo rename gstack-artifacts-Brandon --repo gstack-brain-Brandon --yes
expected the "[HOST/]OWNER/REPO" format, got "gstack-brain-Brandon"

Because stderr was suppressed with 2>/dev/null, this surfaced only as the generic warning gh rename failed (repo may not exist or permission denied) — misdirecting users who actually have ADMIN on the repo. The step is then mark_done'd, so a second /gstack-upgrade run does not retry. The local pointer at ~/.gstack-artifacts-remote.txt ends up referencing a repo that doesn't exist on GitHub until the user notices.

I hit this today upgrading from v1.26.4.0 → v1.32.0.0 on a fully-authenticated gh with ADMIN on the legacy gstack-brain-$USER repo.

What this PR changes

• Resolve the authenticated user's login via gh api user --jq .login.
• Qualify both the existing-name check (gh repo view) and the rename/edit invocations with $OWNER/$REPO.
• Capture stderr from each attempt so the warning prints the real gh error instead of a generic guess.
• Extend the fake gh mock in test/migrations-v1.27.0.0.test.ts so existing tests still drive the rename path with the new gh api user call.

No behavior change for users where the migration would already be a no-op (gh unauthenticated, repo already at the new name, or no legacy state).

Test plan

• bun test test/migrations-v1.27.0.0.test.ts — 11/11 passing locally
• bash -n gstack-upgrade/migrations/v1.27.0.0.sh — clean
• Manually re-ran corrected command on my repo (gh repo rename gstack-artifacts-Brandon --repo agile-operators/gstack-brain-Brandon --yes) — succeeded

🤖 Generated with Claude Code

---

^{Need help on this PR? Tag @codesmith with what you need.}

• Let Codesmith autofix CI failures and bot reviews