docs: add agents.md and agents.yaml for AI agent development setup

.agents/agents.md

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+# react-router - Agent Guide
+
+This is the agent-facing guide for working in this repository.
+
+## Project Context
+- Declarative routing for React
+
+## Setup
+See [`agents.yaml`](./agents.yaml) for prerequisites, commands, and ports.
+
+### Setup Hints
+- `nvm` may not be loaded in your shell. If `nvm: command not found`, source it first.
+- `npm start` runs in the foreground — background it the same way.
+
+## Running the App
+- Clone the repo
+- Install: `npm install`
+- Dev: `npm start &`
+- Access: `http://localhost:3000`
+
+## Architecture
+- React routing library (v3)
+- Entry point: `modules/index.js`
+
+## Working Agreement
+- Write modular code
+- Keep testable
+- Incremental validation
+- Commit meaningful checkpoints
+- Write tests
+- Validate before handoff

.agents/agents.yaml

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+repo:
+  name: react-router
+  description: Declarative routing for React
+  owner: ReactTraining
+
+prerequisites:
+  - name: GitHub CLI
+  - name: npm
+  - name: node
+    version: '>=12.0.0'
+
+setup:
+  - npm install
+  - npm start
+
+dev:
+  command: npm start
+  port: 3000
+
+build:
+  command: npm run build
+
+test:
+  command: npm test
+
+lint:
+  command: npm run lint