fix: LSP command resolution and ENOENT crash on Windows/MSYS (#901) (#925)

Two fixes:

1. lsp/config.ts: Use `where.exe` instead of `which` on Windows.
   MSYS's `which` returns POSIX paths (/c/Users/...) that Node's
   spawn() can't execute. `where.exe` returns native Windows paths.

2. lsp/client.ts: Handle spawn ENOENT error gracefully. When the LSP
   server binary doesn't exist, the error event now triggers a clean
   exit instead of bubbling up and crashing auto-mode.

packages/pi-coding-agent/src/core/lsp/client.ts

@@ -437,6 +437,14 @@ export async function getOrCreateClient(config: ServerConfig, cwd: string, initT
 			env: env ? { ...process.env, ...env } : undefined,
 		});
 
+		// Handle spawn failure (e.g., ENOENT when the command doesn't exist).
+		// Without this, the error bubbles up and can crash auto-mode (#901).
+		proc.on("error", (err: NodeJS.ErrnoException) => {
+			if (err.code === "ENOENT") {
+				proc.emit("exit", 1);
+			}
+		});
+
 		const exitedPromise = new Promise<number>((resolve) => {
 			proc.on("exit", (code: number | null) => resolve(code ?? 1));
 		});

packages/pi-coding-agent/src/core/lsp/config.ts

@@ -177,9 +177,16 @@ const LOCAL_BIN_PATHS: Array<{ markers: string[]; binDir: string }> = [
 ];
 
 function which(command: string): string | null {
-	const result = spawnSync("which", [command], { encoding: "utf-8" });
+	// On Windows, prefer `where.exe` over `which` — MSYS/Git Bash's `which`
+	// returns POSIX paths (/c/Users/...) that Node's spawn() can't execute.
+	// `where.exe` returns native Windows paths (C:\Users\...).
+	const isWindows = process.platform === "win32";
+	const cmd = isWindows ? "where.exe" : "which";
+	const result = spawnSync(cmd, [command], { encoding: "utf-8", shell: isWindows });
 	if (result.status !== 0) return null;
-	return result.stdout.trim() || null;
+	// `where.exe` may return multiple lines — take the first
+	const resolved = result.stdout.trim().split(/\r?\n/)[0]?.trim();
+	return resolved || null;
 }
 
 export function resolveCommand(command: string, cwd: string): string | null {