feat(coding-agent): type ToolCallEvent.input per tool

Matches ToolResultEvent pattern with typed inputs via discriminated union.

- Export *ToolInput types from tool schemas
- Add *ToolCallEvent interfaces for each built-in tool
- Add isToolCallEventType() guard with overloads for built-ins

Direct narrowing (event.toolName === "bash") doesn't work due to
CustomToolCallEvent.toolName: string overlapping with literals.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/coding-agent/src/core/tools/grep.ts

@@ -1,6 +1,6 @@
 import { createInterface } from "node:readline";
 import type { AgentTool } from "@mariozechner/pi-agent-core";
-import { Type } from "@sinclair/typebox";
+import { type Static, Type } from "@sinclair/typebox";
 import { spawn } from "child_process";
 import { readFileSync, statSync } from "fs";
 import path from "path";
@@ -29,6 +29,8 @@ const grepSchema = Type.Object({
 	limit: Type.Optional(Type.Number({ description: "Maximum number of matches to return (default: 100)" })),
 });
 
+export type GrepToolInput = Static<typeof grepSchema>;
+
 const DEFAULT_LIMIT = 100;
 
 export interface GrepToolDetails {