docs: RULES.md states the branch-per-issue model (closes #193)

Replaces the "merge to main when done" checklist line with the wording
sneak approved on the issue.

Model: opus-5-5

RULES.md

@@ -108,7 +108,10 @@ contradicts either, the originals govern.
 - [ ] All tool invocations via `make` targets, never directly
 - [ ] `make check` = `make test` + `make lint` + `make fmt-check`
 - [ ] `main` always passes `make check`
-- [ ] Feature branches for all changes, merge to main when done
+- [ ] Branch per issue, cut from `next`. One PR per issue, based on `next`,
+      squash-merged into `next` on a passed independent review. `main` is
+      reached only by the per-milestone PR from `next`; releases are tagged from
+      `main`. `next` and `main` must stay green; feature branches may be red.
 - [ ] No force-push to `main`
 - [ ] Pre-commit hook runs `make check`