[Presets] Support for dstack preset push and dstack preset pull

[peterschmidt85]

Push and pull

A preset created on one machine can now be used on another. Push it to the registry:

$ dstack preset push qwen38-27b-mi300x main/qwen38-27b-mi300x
OK

Then, on any other machine, pull it from the registry and export it to a service configuration:

$ dstack preset pull main/qwen38-27b-mi300x
OK

$ dstack preset export main/qwen38-27b-mi300x -f qwen38.dstack.yml
OK

$ dstack apply -f qwen38.dstack.yml

export writes the preset's patch files next to the configuration, so the service is applied
exactly as it was verified.

Push shares the verified service, its benchmark, and the hardware it was verified on. The
configuration, prompt, and trials that produced it stay on the machine that created it.

A pulled preset is used like any other, and is named <project>/<name>, which can never clash
with a preset of your own:

$ dstack preset list -a
 NAME                      ID                                    BASE              CONSTRAINTS      BENCHMARK                             STATUS           SUBMITTED
 main/qwen38-27b-mi300x    e7c8b170-8ca6-41cc-bfa9-ed1ba71c8c88  Qwen/Qwen3.8-27B  io=10K/1.5K c=4  tps/user=138 ttft=852ms ctx=1M        pulled           11 mins ago
 qwen38-27b-mi300x-crack2  6900d9d7                              Qwen/Qwen3.8-27B  io=10K/1.5K c=4  tps/user=138 ttft=852ms ctx=1M  ████  verified (4/15)  1 week ago

Its status is pulled, in the table above and in dstack preset get --json.

Pushing the same name again pushes a new preset that the name then points to; the previous one
stays available as <project>/<id>, which pull also accepts.

Registry

The registry is part of the dstack server. On dstack Sky, that is https://sky.dstack.ai. Push and
pull require membership in the project. If the server has no registry, both say so.

The server comes from the project's entry in ~/.dstack/config.yml. A project that is not there
falls back to dstack Sky; DSTACK_NO_SKY_FALLBACK turns the fallback off.

Note

The fallback is exactly the one dstack login applies.

Also here

• dstack preset export no longer fails with IsADirectoryError when the preset mounts a
  directory in files.
• Presets can be the target of events, and list_events filters by them, so a server with a
  registry can record who pushed what. The events table shows them and filters by them.

Breaking changes

• dstack preset get --json reports the served model repository as repo instead of model, so
  base (the model family), repo (what the service loads), and service.model (what the
  endpoint serves it as) are now distinct.

Backward compatibility

• Presets created by earlier versions are upgraded when read.

Note

The registry that push and pull talk to is implemented separately and is not part of this
PR.