Feature: support spec.json as default alongside spec.yaml

[roebi]

Feature: support spec.json as default alongside spec.yaml

Current behaviour

~/.openapi/spec.yaml is the only default path checked.
Renaming spec.json → spec.yaml works but is misleading.

Desired behaviour

Auto-detect both defaults in order:

1. ~/.openapi/spec.yaml
2. ~/.openapi/spec.json

First found wins. --spec flag always overrides.

Labels

enhancement