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/configFile.ts

@@ -27,8 +27,8 @@
 
 import { existsSync, readFileSync, statSync } from "node:fs";
 
-import { CliError } from "./cli.ts";
 import { defaultConfigPath, type ConfigOverrides } from "./config.ts";
+import { CliError } from "./errors.ts";
 
 const ALLOWED_KEYS: ReadonlySet<string> = new Set<string>([
     "moveInterval",