Migrate to Pi Coding Agent SDK for multi-provider LLM support

Replace the custom Pi API fetch client with the @earendil-works/pi-coding-agent
SDK, enabling support for multiple LLM providers (Anthropic, OpenAI, DeepSeek,
Google Gemini, xAI/ZAI) while maintaining backward compatibility.

Key changes:
- Add piSdk.ts with SDK session management and provider auto-detection
- Refactor client.ts to delegate to SDK adapter, keeping public API surface
- Update documentation to reflect multi-provider environment variables
- Add RPC contracts for LLM model selection and provider configuration
- Update agent runner to support provider-specific tools and parameters

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pnpm-workspace.yaml

@@ -3,7 +3,10 @@ packages:
   - "packages/*"
 
 allowBuilds:
+  '@google/genai': true
   better-sqlite3: true
   electron: true
   esbuild: true
-  puppeteer: set this to true or false
+  koffi: true
+  protobufjs: true
+  puppeteer: true