refactor/ migrate from OpenAI to OpenRouter integration

Major improvements to AI integration and developer experience:

## OpenRouter Migration
- Replaced @ai-sdk/openai with @openrouter/ai-sdk-provider
- Updated chat API route to use OpenRouter for access to 100+ AI models
- Changed environment variables from OPENAI_* to OPENROUTER_*
- Updated default model from gpt-5-mini to openai/gpt-5-mini
- Benefits: Users can now access multiple AI providers (OpenAI, Anthropic, etc.) through a single unified API

## Documentation Overhaul
- Expanded CLAUDE.md from 3 lines to comprehensive 225-line guide
- Added complete project structure, tech stack overview, and AI integration guidelines
- Documented all environment variables, scripts, and common tasks
- Added critical rules for AI assistants working with the codebase
- Enhanced checkpoint command with detailed step-by-step instructions

## README Updates
- Updated feature list to highlight OpenRouter integration
- Revised setup instructions for OpenRouter API key acquisition
- Updated environment variable examples and deployment configuration
- Changed all OpenAI references to OpenRouter throughout documentation

## Create Command Updates
- Bumped create-agentic-app version from 1.0.0 to 1.0.1
- Synchronized all template files with main project documentation
- Updated template env.example and package.json with OpenRouter configuration

## Dependency Updates
- Added @openrouter/ai-sdk-provider@^1.2.0
- Updated @ai-sdk/openai from ^2.0.53 to ^2.0.60
- Updated @ai-sdk/react from ^2.0.78 to ^2.0.86
- Updated ai from ^5.0.78 to ^5.0.86
- Updated better-auth from ^1.3.29 to ^1.3.34
- Updated drizzle-kit from ^0.31.5 to ^0.31.6
- Updated eslint from ^9.38.0 to ^9.39.0
- Various type definition updates

Breaking Changes:
- Projects must update environment variables from OPENAI_* to OPENROUTER_*
- API keys must be obtained from openrouter.ai instead of platform.openai.com
- Model names now follow OpenRouter format (e.g., "openai/gpt-5-mini")

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/app/api/chat/route.ts

@@ -1,11 +1,11 @@
-import { openai } from "@ai-sdk/openai";
+import { openrouter } from "@openrouter/ai-sdk-provider";
 import { streamText, UIMessage, convertToModelMessages } from "ai";
 
 export async function POST(req: Request) {
   const { messages }: { messages: UIMessage[] } = await req.json();
 
   const result = streamText({
-    model: openai(process.env.OPENAI_MODEL || "gpt-5-mini"),
+    model: openrouter(process.env.OPENROUTER_MODEL || "openai/gpt-5-mini"),
     messages: convertToModelMessages(messages),
   });