refactor(sync): extract SyncJob for session sync orchestration

[samzong]

What's changed?

• Extract SyncJob / SyncStats / ExistingState from the monolithic run_sync_job_inner loop in src/sync.rs
• Split adapter scan, existing-state load, per-session process, backfill, and progress reporting into methods with the same control flow

Why

• The previous function mixed adapter iteration, scan, persistence, and reporting in one long body, making incremental sync hard to follow and change safely
• Structure matches recent adapter helper extractions without changing sync semantics

Verification

• cargo test --lib sync — 34 passed (sync unit tests, adapter scan_for_sync, integration regression sync cases, related import/TUI filters)