Enhance the CLI error output with actionable recovery suggestions by adding a 'Hint string' field to 'api.APIError' and populating hints for common failures.
## What Add actionable recovery suggestions to CLI error output. Currently some hints exist but are baked into `err.Error()` strings informally. - [ ] Add `Hint string` field to `api.APIError` - [ ] Define a `HintedError` interface so any error type can carry a hint - [ ] Populate hints in `internal/api/errors.go` for common failures (e.g., 401 → `"Run: gumroad auth login"`, 404 → `"Check the resource ID and try again"`, rate limit → `"Wait a moment and retry"`) - [ ] Update `classifyCommandError` in `internal/cmd/errors.go` to surface hints - [ ] In human mode, print the hint as a dimmed line below the error message - [ ] In `--json` mode, include `"hint"` in the error envelope ## Why clig.dev recommends: "Catch expected errors and rewrite them for humans — suggest the next step." Right now, a 401 prints a generic error. With hints, it tells you exactly what to run to fix it. This is a small change with outsized impact on the first-run experience.