feat(cloud): add idempotent 'jcode cloud sessions sync'

Scan ~/.jcode/sessions (or --sessions-dir), upload new/changed sessions
to Jade, and track per-session sha256 state in cloud_sessions_sync.json
(0600) so reruns are idempotent and safe to schedule. Supports
--dry-run, --since-days (default 7), --all, --max (default 50), --force,
--raw, and --json. Skips non-session files (req.json, journals, etc).
This is the building block for automatic/scheduled background sync.