envit — repo context for agents

envit is a virtualenv for repo context. The project's envit.json declares which git repositories an agent should be able to read; envit sync materializes them, read-only and commit-pinned, under .envit/repos/. The bytes live once per machine in a shared store — never edit them, and never commit .envit/ (it's gitignored; the manifest and envit.lock.json are committed instead).

Reading context (the common case — no commands needed)