docs: require a rebase only when the branch has merge conflicts (#3239)

The pre-submit checklist in `AGENTS.md` required every branch to be
rebased onto `main`. A rebase without a conflict adds no information and
forces a new revision on each `main` update.

This change requires only that the branch has no merge conflicts with
`main`. A rebase is required when a conflict exists.

AGENTS.md

@@ -73,7 +73,8 @@ Complete every item before you upload a PR or push a new revision.
       fixes that the diff does not contain.
 - [ ] The PR description names new functions that have no production caller
       yet, or the PR removes them.
-- [ ] The branch is rebased onto `main` and has no merge conflicts.
+- [ ] The branch has no merge conflicts with `main`. Rebase only when a
+      conflict exists.
 - [ ] `npm run gen` ran after a change to a tool schema, a tool description,
       or a CLI option. The generated files are committed.
 - [ ] `npm run format` ran. No line is longer than 80 columns.