docs: make enforce_admins flip required in exit checklist (#93 review)

Code review on #101 pushed back on step 4 of the "When the exemption
ends" checklist: "Optionally flip enforce_admins to true". Leaving it
false in a 2-person setup keeps the admin-bypass door open even after
the single-owner workaround is no longer needed — which defeats the
point of having an exit checklist.

Drops "Optionally" and adds a one-line rationale so a future reader
understands why the flip is non-optional.

Refs #93

Author: constk

CONTRIBUTING.md

@@ -60,7 +60,7 @@ gh pr merge <N> --admin --squash --delete-branch
 1. Drop the `--admin` flag from the merge command and adopt standard PR review.
 2. Remove this entire subsection.
 3. Update `CODEOWNERS` to add the new collaborator.
-4. Optionally flip `enforce_admins` to `true` in the branch-protection JSON for both branches.
+4. Flip `enforce_admins` to `true` in the branch-protection JSON for both branches. Leaving it `false` would keep the admin-bypass door open even after the single-owner workaround is no longer needed — defeats the point of removing the workaround.
 
 All four changes land in a single PR.