feat: /login command and auto-model selection for Altimate backend

## Feature request

- [x] I have verified this feature doesn't already exist in the codebase

### Description

Currently users must manually navigate the provider selection and model picker workflows even when using the Altimate backend. This adds friction for teams that just need to enter their Altimate credentials and start working.

**Proposed changes:**
- `/login` slash command to set up Altimate credentials (URL, tenant, API key)
- Auto-select the Altimate model when credentials are configured — skip provider/model dialogs
- Single "Altimate AI" model entry (backend handles model routing via litellm)
- Use configured URL instead of hardcoded localhost