Extract CliError into src/errors.ts

CliError used to live in cli.ts and was imported by configFile.ts purely
to grab a one-line class — the dependency arrow said 'config-file loader
depends on the CLI parser' when really it just needed a shared error
type.

Moving CliError to its own errors.ts module flattens the graph:

  errors.ts  (no internal deps)
    |
    +-- cli.ts
    +-- configFile.ts
    +-- config.ts  (will use it in the next commit)

cli.ts re-exports CliError for any consumer that prefers to keep
importing it from there; the canonical home is now errors.ts.

src/move.ts

@@ -49,7 +49,13 @@ try {
 }
 
 if (cliArgs.help) {
-    printHelp();
+    // printHelp() resolves defaultConfigPath(), which can throw CliError
+    // when neither $XDG_CONFIG_HOME nor $HOME is set.
+    try {
+        printHelp();
+    } catch (err: unknown) {
+        failUser(err);
+    }
     process.exit(0);
 }
 if (cliArgs.version) {