feat(model): split provider and model selection

[Mineru98]

Problem

Provider choice and model choice are currently blended together. That makes it hard for users to understand whether they are choosing a runtime account, a model family, or a specific model variant.

Proposed change

• Separate provider selection from provider-specific model selection.
• Keep the TUI status and command discovery aligned with the new split.
• Preserve existing command behavior while making provider/model state explicit.

Acceptance criteria

• Users can switch providers and provider models without losing the selected runtime meaning.
• Model/status display reflects the provider/model split consistently.
• Command discovery exposes the same split so help output matches the TUI.

Verification

• git diff --check
• mix format --check-formatted on changed Elixir files
• mix test test/ourocode/model test/ourocode/provider/codex_client_test.exs test/ourocode/command/registry/builtin_test.exs test/ourocode/terminal/command_model_commands_test.exs test/ourocode/terminal/command_discovery_commands_test.exs test/ourocode/terminal/event_loop_command_dispatch_test.exs test/ourocode/terminal/model_status_test.exs test/ourocode/terminal/tui_chat_test.exs test/ourocode/terminal/tui_model_selection_test.exs test/ourocode/terminal/tui_state_test.exs test/ourocode/terminal/tui_submit_test.exs -> 94 passed

Closes #47
Closes #51