Configurable error response verbosity (verbose / standard / limited)

## Request

Allow operators to configure how much error detail is returned in HTTP responses:

- **`verbose`** — full stack traces (default for dev mode)
- **`standard`** — current behavior: `error.toString()` (constructor + message)
- **`limited`** — for non-500 errors without a status code, return only the error code/constructor; otherwise use `standard`

Useful for production deployments where internal error details should not be exposed to clients.

🤖 Filed by Claude on behalf of Kris.