fix: support cross-backend cheap provider for smart routing

[swagasaurus]

Summary

Support cross-backend cheap provider resolution for smart routing. When LLM_CHEAP_BACKEND differs from LLM_BACKEND, the cheap provider is now resolved independently through the provider registry instead of cloning the primary backend's config.

Fixes a bug where create_cheap_provider_for_backend() always used the primary backend's RegistryProviderConfig and only swapped the model name — meaning LLM_CHEAP_BACKEND=ollama with LLM_BACKEND=anthropic would route hermes3 through Anthropic's API.

Root Cause

LLM_CHEAP_BACKEND / llm_cheap_backend DB setting had zero references in Rust source. The setting was stored but never consumed by LlmConfig, so create_cheap_provider_for_backend() had no way to resolve a different provider.

Changes

src/llm/config.rs

• Add cheap_backend: Option<String> and cheap_provider: Option<RegistryProviderConfig> fields to LlmConfig

src/settings.rs

• Add llm_cheap_backend: Option<String> to Settings (DB/serde deserialization)

src/config/llm.rs

• Wire LLM_CHEAP_BACKEND env var and llm_cheap_backend DB setting into config resolution
• Resolve cheap provider through custom providers (llm_custom_providers) and built-in registry
• Priority: DB setting > env var

src/llm/mod.rs

• In create_cheap_provider_for_backend(), check cheap_backend first
• When set and differs from primary, use the independently resolved cheap_provider
• Falls back to existing behavior when cheap_backend is unset or matches primary

src/llm/models.rs

• Add new fields to test config defaults

Backward Compatible

When cheap_backend is unset or matches the primary backend, existing behavior is preserved exactly. The new code path only activates when cross-backend routing is explicitly configured.

Testing

Tested with:

• Primary: anthropic / claude-opus-4-6
• Cheap: custom provider home (Ollama adapter at http://127.0.0.1:11434) / hermes3
• Verified logs show cheap provider resolving through Ollama instead of Anthropic

[standardtoaster]

The new code path (cheap_backend != backend in create_cheap_provider_for_backend) has no test coverage -- the testing section only shows manual log inspection.

I'd like to see more testing. Unit tests:

• cheap_backend set and differs from primary -- provider uses cheap_provider config, not primary
• cheap_backend set but matches primary -- falls back to existing clone behavior
• cheap_backend set to unknown provider name -- falls back to primary with warning
• cheap_backend set to nearai -- routes through NearAI config, not registry
• cheap_backend unset -- existing behavior unchanged

For manual verification without cloud keys: LLM_CHEAP_BACKEND must differ from LLM_BACKEND to trigger the new path. Set up a custom provider in llm_custom_providers, point LLM_CHEAP_BACKEND at its ID, and check for "Creating cross-backend cheap provider" in RUST_LOG=ironclaw::llm=debug output.

Nits:

• NearAI backend detection (nearai | near_ai | near) is duplicated between config/llm.rs:310 and llm/mod.rs:496. Extract a helper to prevent drift.
• if let Some(_def) = registry.find(&cb_lower) discards the result then resolve_registry_provider looks it up again. Minor since it's startup-only.