Silently accepts contradictory flag combinations (--auto + -o/--output, --auto + --check)

## Summary

`--auto` is a preset that implies `--inplace` (which, per `--help`, "ignores --output"). Combining `--auto` with `--output`/`-o`, or with `--check`, is contradictory — but flowmark accepts the combination silently instead of erroring, which produces confusing no-ops.

### `--auto` + `-o -` / `--output`

```
$ flowmark --auto -o - FILE
# exit 0, nothing on stdout, FILE unchanged, no error/warning
```

Because `--auto` ⇒ `--inplace` and `--inplace` ignores `--output`, this writes nothing to stdout. It is easy to mistake the empty stdout for "flowmark produced an empty document" when piping into another tool or a diff — it sent me down a long, wrong debugging path before I realized `-o` was being silently dropped.

### `--auto` + `--check`

```
$ flowmark --auto --check FILE
```

`--auto` means "format in place"; `--check` means "don't write anything." The combination is contradictory, but it runs silently as a check (the in-place intent is dropped). It is also what a Makefile/CI naturally ends up writing (`flowmark --auto --check .`) when mirroring `flowmark --auto .`, which hides the contradiction.

## Request

Emit a clear, consistent error (or at least a warning) for incompatible flag combinations — `--auto` together with `--output`/`-o`, and `--auto` together with `--check` — instead of silently ignoring one side. Predictable errors on contradictory flags would prevent the silent no-ops above.

## Note

The same flag handling is present in flowmark-py 0.7.2 (filed separately; link to follow), so this likely belongs in the shared CLI design.