feat: add smart routing provider for cost-optimized model selection (#281)

* feat: add smart routing provider for cost-optimized model selection

Route simple tasks (greetings, status checks, short questions) to a cheap
model (e.g. Haiku) and complex tasks (code generation, analysis) to the
primary model, reducing agent costs without sacrificing quality.

Activates automatically when NEARAI_CHEAP_MODEL is set. Cascade mode
retries uncertain cheap-model responses with the primary model.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: apply cargo fmt formatting

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* refactor: extract provider chain into shared build_provider_chain()

Consolidate the duplicated LLM provider chain construction from main.rs
and app.rs into a single build_provider_chain() function in llm/mod.rs.

This fixes the inconsistency where app.rs was missing retry wrapping
that main.rs had, and ensures both paths apply identical decorators:
retry → smart routing → failover → circuit breaker → cache.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix: address PR review — uncertainty detection and clippy lint

- Remove false-positive short response (<20 chars) uncertainty check
  that would escalate "Yes.", "42" etc. Now only empty responses and
  explicit uncertainty phrases trigger cascade escalation.
- Add #[allow(clippy::type_complexity)] to build_provider_chain() to
  fix CI clippy -D warnings failure.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/setup/wizard.rs

@@ -1176,6 +1176,7 @@ impl SetupWizard {
                 response_cache_max_entries: 1000,
                 failover_cooldown_secs: 300,
                 failover_cooldown_threshold: 3,
+                smart_routing_cascade: true,
             },
             openai: None,
             anthropic: None,