Make command discovery reflect the provider and model split

[Mineru98]

Problem

After provider selection and model slug selection are split, command discovery also needs to explain the split. If /help, /commands, and command dispatch still treat /model as the backend picker, users will see one behavior in the UI and another in the command list.

Proposed change

Register /provider as the backend/provider picker and make /model the provider-specific model command. Help and command discovery should list both commands with summaries that match their new behavior.

Acceptance criteria

• /provider and /providers are registered as provider/backend selection commands.
• /model and /models are registered as provider-specific model listing/selection commands.
• Command dispatch passes command arguments through to the model command renderer.
• /help and /commands include both commands in the expected order.
• Existing slash command discovery remains deterministic.

Related files

• lib/ourocode/command/registry/builtin.ex
• lib/ourocode/terminal/command_action_dispatcher.ex
• lib/ourocode/terminal/command_discovery_commands.ex
• lib/ourocode/terminal/tui_submit.ex