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/config.ts

@@ -21,6 +21,8 @@
 
 import { join } from "node:path";
 
+import { CliError } from "./errors.ts";
+
 // Single source of truth for default values. The same file ships in the
 // install tree and is copied to $XDG_CONFIG_HOME/move/config.json on a
 // fresh install (only if no config exists there yet). Values use the CLI
@@ -141,7 +143,17 @@ export function defaultConfigPath(): string {
     if (xdg && xdg.length > 0) {
         return join(xdg, "move", "config.json");
     }
-    const home = process.env.HOME ?? "";
+    const home = process.env.HOME;
+    if (!home || home.length === 0) {
+        // Neither var is set; we don't have a sensible fallback. Throwing
+        // CliError lets the entry-point's normal handler surface this as
+        // 'move: cannot resolve default config path: ...' + exit 2 instead
+        // of silently producing '/.config/move/config.json' and bewildering
+        // the user with a downstream 'file not found' message.
+        throw new CliError(
+            "cannot resolve default config path: neither $XDG_CONFIG_HOME nor $HOME is set",
+        );
+    }
     return join(home, ".config", "move", "config.json");
 }