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/server/models.ts

@@ -128,7 +128,7 @@ export async function discoverPiModels(input: {
   }
   if ((result.exitCode ?? 1) !== 0) {
     const detail = firstNonEmptyLine(result.stderr) || firstNonEmptyLine(result.stdout);
-    throw new Error(detail ? "`pi --list-models` failed: ${detail}" : "`pi --list-models` failed.");
+    throw new Error(detail ? `\`pi --list-models\` failed: ${detail}` : "`pi --list-models` failed.");
   }
 
   return sortModels(dedupeModels(parseModelsOutput(result.stdout)));