Context

Follow-up from #13184 / PR #13185. Review feedback from PR #13171 requested replacing real git usage in publish tests with run.CommandStubber.

This was deferred because it touches 20+ test cases and is better handled as a focused refactor.

Task

Convert all real exec.Command("git", ...) usage in pkg/cmd/skills/publish/publish_test.go to use run.Stub() / run.CommandStubber:

• Replace initGitRepo helper (creates real git repos with bare remotes) with command stubs for git remote -v, git config --get-regexp, git remote get-url, etc.
• Replace newTestGitClientWithUpstream helper with stubs for git symbolic-ref, git rev-list, git push
• Replace runGitInDir helper and all direct exec.Command calls
• Convert TestEnsurePushed test cases to verify correct git commands are called via stubs
• Convert inline git init/config/check-ignore calls in test setups

Prior art

• pkg/cmd/pr/create/create_test.go lines 1580-1589 — run.Stub() setup
• pkg/cmd/pr/create/create_test.go lines 737-746 — cs.Register examples