feat(models): make the provider model catalog an overlay on the presets

Rework the per-provider model catalog from a wholesale replacement into an
overlay stored at <workspace>/system/models.json, so it no longer bloats
config.json and untouched presets keep following the code-side metadata.

- Store only overrides + hidden tombstones; the editor prefills the full
  effective list (presets - removals + overrides) so adding one model can no
  longer wipe the rest, and only the diff from the presets is persisted.
- Migrate an early adopter's legacy config.json provider_model_catalog into
  the overlay store once, then strip the old key.
- Share the family budget table via resolve_family_spec so the editor shows
  the same context window / max output the runtime uses (e.g. gpt-6-astra).
- UI: widen the vendor modals, fix the vendor picker being clipped by the
  modal's scroll body, go straight to the custom-provider modal once every
  built-in is configured, and simplify the catalog hint copy.

Co-authored-by: cowagent <cow@cowagent.ai>