Allow PR review mode override for third-party platforms (e.g., via thirtparty MCP etc Bitbucket ...)

I'm using a third-party Bitbucket MCP to bring PR comments into plannotator.

Currently the rich PR review mode (header PR buttons, Post Comments, native thread UI, viewed sync, etc.) only activates for GitHub/GitLab — `parsePRUrl()` rejects other URLs and `prMetadata`
stays null.

External Annotations API is great for inline injection, but PR mode UI is locked behind the built-in providers.

Request: provide an override mechanism so third-party integrations (MCP servers, custom tools) can activate PR review mode with their own data — similar to how External Annotations API
lets external sources push inline comments.

Happy to contribute a PR if there's a preferred direction.