fix(cli): persist backfill checkpoints

[xingkaixin]

Summary

• enable durable checkpoints for full and source backfill operations
• persist each finalization cursor emitted by the scan worker
• keep stale sessions before a restored cursor eligible for re-finalization

Root cause

Backfill scheduling read the stored cursor but never requested durable worker checkpoints or installed a checkpoint callback. As a result, markAgentFullSyncProgress had no production caller and interrupted backfills always restarted finalization from the beginning.

Verification

• added an engine test for checkpoint wiring and cursor persistence
• extended the worker integration test to cover stale sessions before the cursor
• pnpm --filter codesesh lint
• pnpm --filter codesesh format:check
• pnpm --filter codesesh test
• pnpm --filter codesesh build

Issue: CS-188