Add Claude update command for rebasing current branch (#88)

- Document the update workflow using `git fetch` and rebase onto the current branch
- Tell Claude to report the result back to the user

Author: BunsDev

.claude/commands/update.md

@@ -0,0 +1,9 @@
+Run the following command:
+
+```
+git fetch origin && git rebase origin/${branch}
+```
+
+Where `${branch}` is the current git branch (run `git branch --show-current` to determine it).
+
+Report the result to the user.