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>

package.json

@@ -45,4 +45,4 @@
     "vitest": "^3.2.4",
     "wait-on": "^8.0.3"
   }
-}
+}