feat(sync): fetch remote branch before creating worktree when it exists on origin

[EndersonPro]

Closes #24

Summary

• When SyncWithRemote is enabled and the target branch doesn't exist locally but does exist on origin, fetch it and create the worktree from the remote tracking branch
• This applies to both root and package repositories, ensuring consistent branch synchronization across all repos in the worktree
• Add RemoteBranchExists method to GitPort interface and Gateway implementation

Changes

File	Change
internal/app/ports.go	Add RemoteBranchExists to GitPort interface
internal/infra/git/git.go	Implement RemoteBranchExists with fetch + rev-parse check
internal/app/create_service.go	Add remote branch check in createPlannedWorktree() before creating new branch
internal/app/create_service_test.go	Add test for sync with remote branch existing on origin
internal/infra/git/git_test.go	Add test for RemoteBranchExists
internal/app/add_repo_service_test.go	Update mock to include new interface method
internal/app/services_test.go	Update mock to include new interface method

Test Plan

• All 141 tests pass across 11 packages (4 new tests added)
• Build succeeds without errors
• No breaking changes to existing sync behavior
• Existing branches still sync correctly (unchanged path)
• New branches from base still work when no remote branch exists