Implement incremental sync with change tracking for Bushel

[leogdion]

Overview

Add --incremental flag to sync only new/changed data instead of creating duplicates on every sync.

Current Behavior

• Every sync creates new CloudKit records
• No detection of existing records
• Results in duplicate data after multiple syncs

Proposed Enhancement

Features

• --incremental flag for sync command
• Last sync timestamp tracking (stored locally)
• Query existing CloudKit records before sync
• Compare local data with CloudKit data
• Only upload new/changed records

Implementation Steps

1. Add timestamp storage (.bushel-last-sync or similar)
2. Implement CloudKit query for existing records
3. Add comparison logic (by version+build number)
4. Filter out unchanged records before batch upload
5. Update timestamp after successful sync

Benefits

• Reduces unnecessary CloudKit operations
• Prevents duplicate records
• Faster sync times after initial sync
• More production-ready workflow

Example Usage

# First sync - uploads everything
bushel-images sync

# Subsequent syncs - only new data
bushel-images sync --incremental

Notes

• This is mentioned in README limitations section
• Would significantly improve user experience
• Important for production usage