fix(coding-agent): improve bash tool error handling (#479)

- Validate working directory exists before spawning to provide clear error message
- Add spawn error handler to prevent uncaught exceptions when shell not found or cwd invalid
- Add tests for both error scenarios

Without these fixes, spawn errors (e.g., ENOENT from missing cwd or shell) would
cause uncaught exceptions that crash the entire agent session instead of being
returned as clean tool errors.

Co-authored-by: robinwander <robinwander@users.noreply.github.com>

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

@@ -1,5 +1,5 @@
 import { randomBytes } from "node:crypto";
-import { createWriteStream } from "node:fs";
+import { createWriteStream, existsSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import type { AgentTool } from "@mariozechner/pi-agent-core";
@@ -40,6 +40,11 @@ export function createBashTool(cwd: string): AgentTool<typeof bashSchema> {
 		) => {
 			return new Promise((resolve, reject) => {
 				const { shell, args } = getShellConfig();
+
+				if (!existsSync(cwd)) {
+					throw new Error(`Working directory does not exist: ${cwd}\nCannot execute bash commands.`);
+				}
+
 				const child = spawn(shell, [...args, command], {
 					cwd,
 					detached: true,
@@ -119,6 +124,17 @@ export function createBashTool(cwd: string): AgentTool<typeof bashSchema> {
 					child.stderr.on("data", handleData);
 				}
 
+				// Handle shell spawn errors to prevent session from crashing
+				child.on("error", (err) => {
+					if (timeoutHandle) {
+						clearTimeout(timeoutHandle);
+					}
+					if (signal) {
+						signal.removeEventListener("abort", onAbort);
+					}
+					reject(err);
+				});
+
 				// Handle process exit
 				child.on("close", (code) => {
 					if (timeoutHandle) {