Epic: M20 — Provider flexibility

[bug-ops]

Motivation

Adding each new LLM provider as a separate implementation is unsustainable. A single generic OpenAI-compatible adapter covers dozens of APIs with zero new code.

Scope

1. CompatibleProvider — generic OpenAI-compatible adapter with configurable base_url
2. Router provider — fallback chain across multiple providers with health checks

Issues

• Implement CompatibleProvider for OpenAI-compatible APIs #375 Implement CompatibleProvider for OpenAI-compatible APIs
• Add Router provider with fallback chain #376 Add Router provider with fallback chain