Add OpenRouter as a provider

[8crsk]

OpenRouter exposes an OpenAI-compatible API and would give users access to hundreds of models with one key.

Where: app/src/main/java/com/crsk/openclaw/data/providers/ProviderCatalog.kt — the four existing providers (nim/oai/ant/gem) are defined there as CUSTOM openai-completions providers with explicit baseUrl + model lists. Adding one more is mostly following the existing pattern.

Acceptance criteria

• New openrouter entry in ProviderCatalog (baseUrl https://openrouter.ai/api/v1, a few sensible default models)
• Key hint/validation pattern (sk-or-…)
• Shows up in Settings → AI provider and works end-to-end
• ./gradlew testDebugUnitTest lintDebug green

Good first contribution — the provider catalog is a pure-Kotlin, unit-tested file with no Android dependencies.