feat(ai): Create unified AI package with OpenAI, Anthropic, and Gemini support

- Set up @mariozechner/ai package structure following monorepo patterns
- Install OpenAI, Anthropic, and Google Gemini SDK dependencies
- Document comprehensive API investigation for all three providers
- Design minimal unified API with streaming-first architecture
- Add models.dev integration for pricing and capabilities
- Implement automatic caching strategy for all providers
- Update project documentation with package creation guide

tsconfig.json

@@ -4,6 +4,7 @@
 		"noEmit": true,
 		"paths": {
 			"@mariozechner/pi-tui": ["./packages/tui/src/index.ts"],
+			"@mariozechner/pi-ai": ["./packages/ai/src/index.ts"],
 			"@mariozechner/pi-agent": ["./packages/agent/src/index.ts"],
 			"@mariozechner/pi": ["./packages/pods/src/index.ts"]
 		}