model: add agent model list

[hbrooks]

Summary

• Adds an agent model command group (alias agent models) with list (alias ls), backed by the existing ApiClient.listSupportedModels() / GET /v1/models.
• Table prints ID / NAME / DEFAULT, with the account default marked. --json passes the raw payload through, including rate_card.
• The model registry was previously only reachable through the connect composer, so anyone hand-writing model: under claude: in a config YAML had to guess the id.
• Documents the command in README usage and in the ellipsis skill command reference.

Test plan

• bun run typecheck clean
• bun run test (298 pass, 19 files)
• agent model list and agent models ls against the live API return the 5 current models with Opus 5 marked default
• agent model list --json emits the raw array
• agent --help shows model|models; agent model --help shows list|ls
• Bare agent model prints group help and exits 1, matching template and sandbox