Provider rotation strategy: multi-provider failover with quota-aware scheduling

[alexsiri7]

Add provider rotation to Gas City so agents can fail over between providers:

• ProviderStrategy interface with RandomStrategy implementation
• Providers []string and ProviderStrategy fields on Agent/AgentPatch/AgentOverride
• Wire provider rotation into ResolveProvider — agents cycle through configured providers
• Skip session restarts when provider is in quota cooldown
• Re-roll provider on session restart to avoid quota-exhausted cycles
• Startup probe for provider session health
• Test coverage for provider rotation selection

Motivation: when running multiple LLM providers (Claude, Gemini, OpenAI via proxy), agents should automatically rotate away from providers that are rate-limited or down.