- demo-spec.yaml ships as embedded demo data (include_str!)
- --demo loads .squawk.json session, falls back to embedded spec
- demo mode saves to .squawk.json (legacy format)
- normal mode continues to use .squawk.yaml (OpenAPI format)
- remove demo.rs (replaced by embedded spec)

- Add OpenAPI 3.0.3 spec parsing (JSON/YAML) via new src/openapi.rs
- Save/load .squawk.yaml as valid OpenAPI spec with x-squawk-*
extensions for version and writable flag
- Accept file or URL arg: sqk <file-or-url>
- Auto-migrate legacy .squawk.json on load
- Read-only mode: external specs or URL-loaded specs cannot be
edited or saved back (gated by x-squawk-writable)