Add dev/main branch model: PRs target dev, main is curated

Switching to a two-branch workflow: contributors open PRs against `dev`,
and `main` is fast-forwarded to a tested `dev` commit at each release.
This separates "things land in staging" (can move fast) from "things
ship to users" (slow, tested in a browser by the maintainer first).

CONTRIBUTING: add a Branch model section explaining the split + how to
retarget a PR.
PR template: add an explicit "this PR targets dev" checkbox at the top
so it's the first thing a contributor confirms.

End-users cloning the repo will now land on `dev` by default; they can
`git checkout main` if they want the curated branch.

.github/pull_request_template.md

@@ -2,6 +2,10 @@
 
 <!-- One paragraph: what changed and why. "Fixed bug" and "Added feature" are not summaries. -->
 
+## Target branch
+
+- [ ] This PR targets **`dev`**, not `main`. All PRs land in `dev`; `main` is curated by the maintainer at each release. If your PR is on `main` by accident, click "Edit" on this PR and change the base.
+
 ## Linked Issue
 
 <!-- Every PR should be linked to an issue.