test: cover PR/MR platform helpers

[sudorest]

Summary

• add regression coverage for PR/MR URL parsing, labels, display helpers, and CLI selection across GitHub and GitLab
• include self-hosted GitLab parsing coverage to protect the platform routing introduced in feat: GitLab merge request review support #364
• migrate the review editor's open-PR preference to a platform-neutral storage key while preserving reads from the legacy GitHub-specific key

Verification

• bun test packages/shared/pr-provider.test.ts packages/shared/review-core.test.ts

Context

Follow-up to #364 to protect the new GitHub/GitLab review abstraction with focused helper coverage and a small platform-neutral naming cleanup.

[backnotprop]

@sudorest

We're building Plannotator Workspaces, where agent plans, specs, and reviews can be collaborated on and distilled into standards and rules. Your annotations and reviews compound into a golden ruleset planning and review agents can use, and CI/CD can verify against.

Joining the waitlist gets you lifetime free access as an early contributor. https://plannotator.ai/workspaces/. Check the OSS contributor box.

The OSS core isn't changing: same free and open source plugin, always.

Thanks for #426.