Add Router provider with fallback chain

[bug-ops]

Epic: #365

Description

Provider that cascades requests through an ordered list of providers, falling back on errors or rate limits.

Design

[llm]
provider = "router"

[llm.router]
providers = ["claude", "compatible"]
health_check_interval_secs = 60

Tasks

• RouterProvider implementing LlmProvider trait
• Ordered fallback: try first provider, on error try next
• Health check: periodic warmup calls to detect provider availability
• Mark unhealthy providers as down, retry after cooldown
• Respect LlmError::RateLimited as trigger for fallback
• Unit tests with mock providers (healthy, failing, rate-limited)