Default --config if one is found on disk

### 🚀 Feature Request

It'd be nice if running `npx typestat` after creating a `typestat.json` config file didn't require a `--config typestat.json`. Seems redundant. Extra user work.

#### Existing Behavior

No auto-detection of a `typestat.json` is done.

#### Change Proposal

If a `typestat.json` exists on disk, default the `--config` argument to point to it.

Start by locating the argument parsing and configuration-loading paths used by `npx typestat`, then check how an explicit `--config typestat.json` is handled. Implement the default only when `typestat.json` exists on disk, preserving explicit configuration behavior, and verify that running `npx typestat` discovers the file.

Written by the indexing model from the issue text.