Tie git to the OpenSpec lifecycle: branch on proposal (git_propose), commit each finished task (git_commit), and archive by merging the feature branch into main (git_archive). Add the openspec-git skill documenting the conventions and wire the triggers into the proposal, apply and archive step guidance. git_archive now also commits the openspec archive result and merges into main (--no-ff), with best-effort origin push so a remote-less repo still works.