Introduce failRuntime() to mirror failUser() in move.ts

The two stderr-fatal paths used to be asymmetric: failUser() is a named
helper with 'move:' prefix and a 'Try --help' hint, while the runKeeper
catch was an inline 'console.error("Error:", err)' + process.exit(1).

failRuntime() now sits next to failUser(), so the entry-point reads as
'one of two well-named fatal paths.' It also prefers err.stack when
available, giving better diagnostics for nut.js / Accessibility-permission
failures than the previous formatter.

Same observable behavior on the success path; failure messages on the
runtime path are now more debuggable.

src/move.ts

@@ -32,8 +32,13 @@ import { loadConfigFile } from "./configFile.ts";
 import { runKeeper } from "./keeper.ts";
 
 /**
- * Print a CLI-style error and exit 2. Returns `never` so callers can use
- * it without TypeScript flagging "variable might be undefined" downstream.
+ * Print a user-error message and exit 2. Used for anything that comes from
+ * invalid input: unknown CLI flags, bad numbers, malformed or missing
+ * config files, unresolvable default paths. The accompanying "Try 'move
+ * --help'" pointer is appropriate for these cases.
+ *
+ * Returns `never` so callers can invoke it without TypeScript flagging
+ * "variable might be undefined" downstream.
  */
 function failUser(err: unknown): never {
     const msg: string = err instanceof Error ? err.message : String(err);
@@ -41,6 +46,19 @@ function failUser(err: unknown): never {
     process.exit(2);
 }
 
+/**
+ * Print a runtime-failure message and exit 1. Used for anything the user
+ * couldn't have prevented from the command line: nut.js errors, missing
+ * Accessibility permission on macOS, unexpected exceptions from the
+ * keeper loop. Prefers the stack trace when available since these failures
+ * usually need a developer to interpret.
+ */
+function failRuntime(err: unknown): never {
+    const detail: string = err instanceof Error ? (err.stack ?? err.message) : String(err);
+    process.stderr.write(`move: runtime error: ${detail}\n`);
+    process.exit(1);
+}
+
 let cliArgs: ParsedCliArgs;
 try {
     cliArgs = parseCliArgs();
@@ -80,7 +98,4 @@ const cliOverrides: ConfigOverrides = {
 
 const config = resolveConfig(fileOverrides, cliOverrides);
 
-runKeeper(config).catch((err: unknown): void => {
-    console.error("Error:", err);
-    process.exit(1);
-});
+runKeeper(config).catch(failRuntime);