Use dev for development and keep main on tagged releases

Related to #517, which describes how skills.sh follows `main` for unpinned installations. Could this repository use `dev` for ongoing development and reserve `main` for published releases? While reviewing an update to v1.3.1, I found that `main` was already [28 commits ahead of that release](https://github.com/mattpocock/skills/compare/v1.3.1...6fd947921b935b7e1e69293a200400f0fdd5c15f). For unpinned global installations, `npx skills update -g` follows the repository's default branch. The update can therefore include changes beyond the release and changelog the user has reviewed. This was verified through repository history and skills CLI 1.7.1 source inspection; the update itself was not run. Proposed repository workflow: - Develop and integrate unreleased changes on `dev`. - When a release is ready, merge it into `main`, including its version and changelog updates, and tag that release commit. - Keep `main` as the default branch, advancing it only for releases. - Keep version tags such as `v1.3.1` fixed so explicit version selections remain reproducible. This would give users three clear choices with the existing installer: ordinary installs and updates follow the latest released content on `main`; explicit references such as `#v1.3.1` stay on that version; `#dev` opts into development changes. This issue proposes a concrete repository workflow for the problem in #517. It requires no skills.sh changes or new `latest` tag convention. Could this workflow be adopted and documented in the installation/update guidance?