`zero providers` had no way to list the models a provider actually serves, so a
custom OpenAI-/Anthropic-compatible endpoint (e.g. a self-hosted proxy serving a
dozen models) forced users to hand-write a config object per model. Live model
discovery already existed for the TUI picker but had no headless CLI surface.
Add `zero providers models [name] [--json]`: it probes the provider's live
model-listing endpoint (an OpenAI-compatible /v1/models or the Anthropic /models)
via providermodeldiscovery.Discover, using the profile's base URL and resolved
credential (inline -> stored -> env). This uses the raw discovery primitive with
no curated-catalog merge or coding-model filtering, so a custom provider's full
model list is returned. Configure the provider once, run `zero providers models`
to see what it serves, and use any id with `zero exec --model <id>` (Zero passes
unknown model ids through to the provider) -- no per-model config needed.
Wired through a new appDeps.discoverProviderModels hook (default:
providermodeldiscovery.Discover) so the command is unit-tested hermetically.
Fixes#381.
