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/contracts/src/rpc.ts

@@ -204,6 +204,8 @@ export type RpcRequestMap = {
   "earnings.getSchedule": { companyId: string };
   "filing.list": { companyId: string; since?: string };
   "model.get": { companyId: string; tab: string };
+  "model.list": undefined;
+  "model.set": { provider: string; modelId: string; thinkingLevel?: "off" | "minimal" | "low" | "medium" | "high" };
   "model.updateCell": { companyId: string; tab: string; row: number; col: number; value: string };
   "model.createRow": { companyId: string; tab: string; label: string; kind: "actual" | "forecast" | "total"; values?: string[] };
   "model.deleteRow": { companyId: string; tab: string; row: number };
@@ -267,6 +269,8 @@ export type RpcResponseMap = {
   "earnings.getSchedule": { schedule: EarningsSchedule[] };
   "filing.list": { filings: Filing[] };
   "model.get": { headers: string[]; rows: ModelRow[] };
+  "model.list": { models: Array<{ provider: string; modelId: string; name: string; available: boolean }> };
+  "model.set": { ok: boolean; provider: string; modelId: string };
   "model.updateCell": { ok: boolean; affectedCells: string[] };
   "model.createRow": { row: ModelRow; position: number };
   "model.deleteRow": { ok: boolean };