fix: update pi packages to 0.51.0, remove bogus type augmentation

- Update @mariozechner/pi-agent-core, pi-ai, pi-coding-agent, pi-tui to 0.51.0
- Delete src/types/pi-coding-agent.d.ts (declared additionalExtensionPaths which SDK never supported)
- Fix ToolDefinition.execute signature (parameter order changed in 0.51.0)
- Remove dead additionalExtensionPaths from createAgentSession calls

src/agents/pi-tool-definition-adapter.ts

@@ -41,12 +41,10 @@ export function toToolDefinitions(tools: AnyAgentTool[]): ToolDefinition[] {
       execute: async (
         toolCallId,
         params,
+        signal,
         onUpdate: AgentToolUpdateCallback<unknown> | undefined,
         _ctx,
-        signal,
       ): Promise<AgentToolResult<unknown>> => {
-        // KNOWN: pi-coding-agent `ToolDefinition.execute` has a different signature/order
-        // than pi-agent-core `AgentTool.execute`. This adapter keeps our existing tools intact.
         try {
           return await tool.execute(toolCallId, params, signal, onUpdate);
         } catch (err) {
@@ -93,9 +91,9 @@ export function toClientToolDefinitions(
       execute: async (
         toolCallId,
         params,
+        _signal,
         _onUpdate: AgentToolUpdateCallback<unknown> | undefined,
         _ctx,
-        _signal,
       ): Promise<AgentToolResult<unknown>> => {
         const outcome = await runBeforeToolCallHook({
           toolName: func.name,