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.

CONTRIBUTING.md

@@ -2,6 +2,17 @@
 
 Thanks for helping. The project is moving quickly, so the best contributions are focused, easy to review, and easy to test.
 
+## Branch model
+
+Odysseus has two branches:
+
+- **`dev`** — where all PRs land. Things can be in flux here; the merge button gets used freely.
+- **`main`** — what users run. Curated and tested by the maintainer. Fast-forwarded to a stable `dev` commit at each release.
+
+**Open your PR against `dev`, not `main`.** The GitHub "base" dropdown defaults to `dev`. If you opened a PR against `main` by accident, click "Edit" on the PR and change the base — no rebase needed.
+
+End-users cloning the repo will land on `dev` by default. To run the curated/stable version: `git checkout main` after clone.
+
 ## Before You Start
 
 - Search existing issues and pull requests before opening a new one.