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/cli.ts

@@ -27,14 +27,7 @@ import { fileURLToPath } from "node:url";
 import { parseArgs } from "node:util";
 
 import { DEFAULT_CONFIG, defaultConfigPath } from "./config.ts";
-
-/**
- * Thrown when CLI input is invalid (unknown option, missing value, bad
- * number, malformed config file). Distinct from runtime errors so the
- * entry point can exit with code 2 (user error) instead of code 1
- * (runtime failure).
- */
-export class CliError extends Error {}
+import { CliError } from "./errors.ts";
 
 /**
  * Result of `parseCliArgs`. Numeric fields are `undefined` when the user