Cross-Platform SCM Support for Scrum Helper

Scrum Helper currently supports only GitHub and a partially-integrated GitLab, with all platform logic tightly coupled in a 2,190-line monolithic file. This makes adding new SCM platforms nearly impossible without duplicating the entire data-fetching flow. This project refactors Scrum Helper's core into an extensible Provider Pattern architecture by defining a standardized SCMProvider interface that decouples platform-specific API calls from the report generation engine. I will then implement native providers for GitLab (including self-hosted instances), Gitea, and Bitbucket, each returning normalized data through a unified schema. Deliverables: (1) SCMProvider interface with normalized data models, (2) Refactored GitHubProvider with full regression coverage, (3) GitLabProvider with self-hosted baseUrl support, (4) GiteaProvider tested against Codeberg, (5) BitbucketProvider with OAuth 2.0 auth, (6) unified caching and cross-provider error handling, and (7) architecture guide and contributor docs for adding future providers. All 4 platforms will generate identical scrum reports by project end.