The --list-models handler in runRootCommand short-circuited to
listModels() right after Settings.init and modelRegistry.refresh,
exiting before extension loading ran in createAgentSession. As a
result, providers contributed via pi.registerProvider() (from -e
paths or settings.extensions) never appeared in the listing.
Extract a runListModelsCommand entry point in cli/list-models.ts
that loads extensions (CLI -e paths and settings.extensions) into
the supplied ModelRegistry, mirroring sdk.ts's handoff of pending
provider registrations, and then delegates to listModels. The load
is intentionally narrow: no agent loop, no MCP servers, no custom
tools.
Fixes#905
