test: changelog Pro happy-path (posts comment + sets gate) (#174)

Companion to the expired-trial test: stubs oasdiff (--open prints a
/review/ep URL + pending status) and curl (GitHub API), and asserts the
Pro review comment is posted, the oasdiff commit status is set, and the
review URL reaches the step summary. Guards the core Pro funnel action
behavior in CI, complementing the prod e2e.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>