tether 0.1.0a6

Lazy forking: tether new decides working branches and the first writable open creates them (--eager / [new] fork = "eager" for the old behaviour; --pin record objects still fork during new). tether promote: fast-forward each system's base branch to the fork, native three-way merge where the system has one (git, lakeFS, Dolt), refuse with a recipe otherwise; fork points recorded in workspace.toml (export schema_version 2). See CHANGELOG.md.