chore: add AGENTS.md documentation for AI agent development guidance (#820)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: hi-ogawa <[email protected]>

Author: Copilot

AGENTS.md

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+# AI Agent Development Guide
+
+This document provides AI-agent-specific guidance for working with the vite-plugin-react monorepo. For comprehensive documentation, see:
+
+- **[README.md](README.md)** - Repository overview and package links
+- **[CONTRIBUTING.md](CONTRIBUTING.md)** - Setup, testing, debugging, and contribution guidelines
+
+## Quick Reference for AI Agents
+
+### Repository Navigation
+
+This monorepo contains multiple packages (see [README.md](README.md#packages) for details):
+
+- `packages/plugin-react/` - Main React plugin with Babel
+- `packages/plugin-react-swc/` - SWC-based React plugin
+- `packages/plugin-rsc/` - React Server Components ([AI guidance](packages/plugin-rsc/AGENTS.md))
+- `packages/plugin-react-oxc/` - Deprecated (merged with plugin-react)
+
+### Essential Setup Commands
+
+```bash
+pnpm install && pnpm build   # Initial setup (see CONTRIBUTING.md for details)
+pnpm dev                     # Watch mode development
+pnpm test                    # Run all tests
+```

packages/plugin-rsc/AGENTS.md

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+# AI Agent Guide for @vitejs/plugin-rsc
+
+This document provides AI-agent-specific guidance for the React Server Components (RSC) plugin. For comprehensive documentation, see:
+
+- **[README.md](README.md)** - Plugin overview, concepts, and examples
+- **[CONTRIBUTING.md](CONTRIBUTING.md)** - Development setup and testing guidelines
+
+## Quick Reference for AI Agents
+
+### Essential Commands
+
+```bash
+# inside packages/plugin-rsc directory
+pnpm build                          # build package
+pnpm tsc                            # typecheck
+pnpm dev                            # Watch mode development
+pnpm test-e2e                       # Run e2e tests
+pnpm test-e2e basic                 # Test specific example
+```