Session log: /save + /sync multi-user change summaries

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.

.claude/commands/save.md

@@ -72,9 +72,32 @@ Format credentials as:
 
 ## After Saving
 
+Before committing, emit a **Change Summary** block for the user to review:
+
+```
+## Change Summary (this session)
+User: <full_name> (from .claude/identity.json)
+Machine: <HOSTNAME>
+
+Files changed:
+<output of: git status --short>
+
+Stats:
+<output of: git diff --stat HEAD>
+```
+
+Then:
 1. Commit with message: "Session log: [brief description of work done]"
 2. Push to gitea remote (if configured)
-3. Confirm push was successful
+3. After push, emit a **Post-commit Summary**:
+   - New commit SHA + message
+   - Author (from `git log -1 --format='%an <%ae>'`)
+   - Files in the commit (from `git show --stat HEAD`)
+4. Confirm push was successful
+
+### Why the summary
+
+In the multi-user setup, commits can land in `main` from either team member. Always attributing author + files makes it obvious who made what change when someone else pulls the repo. Saves re-reading diffs to figure out "wait, when did that happen?"
 
 ## Purpose