Move interface declarations into per-area Interfaces/ subfolders, merge the
small task-list filter classes into StatusFilter/SmartFlagFilter, and simplify
related services, converters and hub DTO handling.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Add GitService.CheckoutBranchAsync; compare targetBranch to current HEAD
before MergeNoFfAsync and switch when they differ. Returns Blocked if the
branch does not exist. Add three new tests (two service, one GitService).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
