Enhance /save and /sync slash commands to attribute commits by author
so Mike and Howard can see at a glance what the other person did.
- sync.sh: loads identity.json, shows incoming/outgoing commits with
author + age before pull/push, groups by author in final summary
- sync.md: describes the new output format + conflict attribution
- save.md: pre-commit Change Summary block + post-commit Summary
Motivation: repo is now shared across team, `git log` alone made it
hard to see "when did Howard change that?" without hunting.
