docs(plan): implementation plan for provider abstraction + FTP

Four-phase TDD plan: (1) Providers.Abstractions package, (2) wrap GitHub behind
IStorageProvider with adapters (regression-gated by the existing 71 tests),
(3) FTP provider (FluentFTP behind an IFtpConnection seam, TDD with an in-memory
fake), (4) Blazor registry + neutral services + capability-driven UI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 2 people