Add RubyLLM::Contract to Ecosystem

docs/_reference/ecosystem.md

@@ -31,6 +31,7 @@ After reading this guide, you will know:
 * How `RubyLLM::RedCandle` enables local model execution from Ruby
 * How OpenTelemetry instrumentation for RubyLLM provides observability into your LLM applications
 * How to test application code by stubbing responses with `RubyLLM::Test`
+* How `RubyLLM::Contract` adds runtime contracts, model-escalating retries, and regression evals on top of RubyLLM
 * Where to find community projects and how to contribute your own
 
 ## RubyLLM::Schema
@@ -283,6 +284,42 @@ For detailed documentation and examples, visit the [RubyLLM::Tribunal repository
 
 ---
 
+## RubyLLM::Contract
+
+**Contracts and Evals for LLM Outputs**
+
+[`RubyLLM::Contract`](https://github.com/justi/ruby_llm-contract) wraps `RubyLLM::Chat` with input/output contracts, business-rule validation, retry with model escalation, pre-flight cost ceilings, and a regression-eval framework. It catches schema-valid-but-logically-wrong output before it reaches your code.
+
+### Why Use RubyLLM::Contract?
+
+LLMs can return JSON that looks correct - valid shape, right types, right fields - while being silently wrong in ways schema validation alone doesn't catch. You often need:
+
+- Business rules that schema can't express
+- Retry with model escalation when a cheap model's output fails the contract
+- Regression evals with baselines to block prompt regressions in CI
+- Pre-flight cost ceilings so a large input doesn't blow your budget
+
+### Key Features
+
+- Class-based DSL: `prompt`, `output_schema`, `validate`, `retry_policy`, `max_cost`
+- Schema validation via [`RubyLLM::Schema`](https://github.com/danielfriis/ruby_llm-schema) with client-side verification
+- Model escalation on validation failure and pre-flight refusal on cost limits
+- LLM-as-judge checks and a regression eval framework with frozen datasets and baselines
+- Pipeline composition with fail-fast and per-step models
+- RSpec / Minitest matchers (`pass_eval`, `satisfy_contract`, `stub_step`)
+
+`RubyLLM::Contract` is runtime - it gates the LLM call and retries on failure - while [`RubyLLM::Tribunal`](https://github.com/Alqemist-labs/ruby_llm-tribunal) grades outputs at test time; the two compose well in the same project.
+
+### Installation
+
+```bash
+gem install ruby_llm-contract
+```
+
+For detailed documentation and examples, visit the [RubyLLM::Contract repository](https://github.com/justi/ruby_llm-contract).
+
+---
+
 ## RubyLLM::TopSecret
 
 **Automatically filter sensitive information from RubyLLM conversations using Top Secret.**