Add model fetchers for OpenCode Zen (https://opencode.ai/zen/v1/models) and
Go (https://opencode.ai/zen/go/v1/models) APIs. Register opencode-go as a new
provider, load shared credentials from auth.json, add known models with pricing,
and create default agents for both tiers on first boot.
Replace the manual "Add Model" form with a search bar that filters by model
name/ID and paginate the catalog at 25 models per page.