OpenRouter static-API-key provider (https://openrouter.ai/api/v1).

OpenRouter is a multi-provider gateway speaking the OpenAI chat wire,
so no `:api-style` override is needed - svar's default OpenAI
transport handles it. Model names are `vendor/model` slugs
(`anthropic/claude-sonnet-4.5`, `openai/gpt-5.1`, ...).

Auth lifecycle:
  1. `vis providers auth openrouter` persists the key once under
     `~/.vis/openrouter-auth.json` as canonical snake_case JSON
     (`api_key` / `saved_at`) - every JSON this repo writes uses
     snake_case string keys.
  2. Later runs resolve the key from config (`~/.vis/config.edn`,
     written by the TUI), then `OPENROUTER_API_KEY`, then the auth
     file. Config wins so status/limits report the key that model
     calls actually use; the env var beats the file so CI stays
     home-directory-free.
  3. `vis providers status openrouter` shows the source without
     exposing the key.
  4. `vis providers logout openrouter` deletes the persisted key.

Limits come from OpenRouter's `GET /api/v1/key`, which reports the
credits consumed by the key and, for capped keys, the credit limit.