refactor(plugin): make plugin installs transactional (#509)

* feat(plugin): stage installs before promote

* feat(plugin): make remote updates transactional

* fix(plugin): rollback relink failures

* refactor(plugin): unify transactional publish flow

* refactor(plugin): extract publish pipeline helpers

* refactor(plugin): add structured source model

* refactor(plugin): promote lockfile to structured sources

* fix(plugin): preserve lock reads when migration rewrite fails

* fix(plugin): write lockfiles atomically

* test(plugin): make source helper assertions cross-platform

Author: jackwener