feat(pi-local): fix bugs, add RPC mode, improve cost tracking and output handling

Major improvements to the Pi local adapter:

Bug Fixes (Greptile-identified):
- Fix string interpolation in models.ts error message (was showing literal ${detail})
- Fix tool matching in parse.ts to use toolCallId instead of toolName for correct
  multi-call handling and result assignment
- Fix dead code in execute.ts by tracking instructionsReadFailed flag

Feature Improvements:
- Switch from print mode (-p) to RPC mode (--mode rpc) to prevent agent from
  exiting prematurely and ensure proper lifecycle completion
- Add stdin command sending via JSON-RPC format for prompt delivery
- Add line buffering in execute.ts to handle partial JSON chunks correctly
- Filter RPC protocol messages (response, extension_ui_request, etc.) from transcript

Cost Tracking:
- Extract cost and usage data from turn_end assistant messages
- Support both Pi format (input/output/cacheRead/cost.total) and generic format
- Add tests for cost extraction and accumulation across multiple turns

All tests pass (12/12), typecheck clean, server builds successfully.

packages/adapters/pi-local/src/ui/parse-stdout.ts

@@ -34,6 +34,11 @@ export function parsePiStdoutLine(line: string, ts: string): TranscriptEntry[] {
 
   const type = asString(parsed.type);
 
+  // RPC protocol messages - filter these out (internal implementation detail)
+  if (type === "response" || type === "extension_ui_request" || type === "extension_ui_response" || type === "extension_error") {
+    return [];
+  }
+
   // Agent lifecycle
   if (type === "agent_start") {
     return [{ kind: "system", ts, text: "Pi agent started" }];