State the proposal scope where proposals are made

The automation fences commits to questions/*.yaml and aggregates.yaml;
the page and README now say so, instead of promising "any file" and
silently failing after approval.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -112,7 +112,8 @@ entirely in this repo, in this order:
    and its workflow must be updated in lockstep.
 
 5. **Change the system through itself.** `/develop-proposal` takes a
-   proposed replacement for any file in this repo — from a person, or
+   proposed replacement for any content file (`questions/*.yaml` or
+   `aggregates.yaml` — nothing else is in scope) — from a person, or
    eventually a locally-run model, through the same public form. An
    owner approves it on `/develop`; approval triggers the automation
    that branches, commits, opens a PR, and merges only when the same