[codex] fix history auth and sync pagination

[pdurlej]

Summary

• add the missing Authorization: Password ... header to Histories, CreateHistory, and History.Delete
• make History.Items compute LoadedServerIndex from the requested start-index, so pagination works after a stored non-zero cursor
• update Syncer.Sync to request the next page from LoadedServerIndex and add regression coverage

Context

This covers the auth issue already raised in #4 and the sync pagination issue raised in #5. While checking #5, I found the one-line fix still breaks incremental syncs that resume from a stored cursor greater than zero: after fetching start-index=100, it asks for start-index=1. This patch fixes that cursor accounting in Items() as well.

Validation

• go test ./...