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>

packages/shared/package.json

@@ -25,10 +25,12 @@
     }
   },
   "dependencies": {
+    "@earendil-works/pi-coding-agent": "^0.74.0",
     "@mosaiciq/contracts": "workspace:*",
     "better-sqlite3": "^12.10.0",
     "exceljs": "^4.4.0",
-    "pptxgenjs": "^4.0.1"
+    "pptxgenjs": "^4.0.1",
+    "typebox": "^1.1.38"
   },
   "devDependencies": {
     "@types/better-sqlite3": "^7.6.13"