Code: symmetric streaming clone landed

Large-repository clone and push now stream symmetrically end-to-end — big packs no longer buffer whole before transfer.

A long-standing rough edge in the git host — large packfiles buffering fully in memory before transfer — is resolved with a symmetric streaming clone path. Clone and push now move data incrementally in both directions, which makes hosting big repositories on Code practical without memory spikes on the server. Verified end-to-end on the dev build.