Feat/openapi multifile spec

[snow-ghost]

Summary

This PR continues the OpenAPI / Swagger compatibility work in ocli by
adding support for multi-document specs and richer schema-aware command help.

What changed

Multi-document spec support

• Added external $ref resolution across local files and remote documents
• Added support for multi-document OpenAPI / Swagger descriptions that split
  paths, parameters, and request bodies across files
• Resolved external references before caching the normalized spec

Richer schema-aware help

• Extended generated command metadata with schema hints
• Added enum, default, nullable, and oneOf hints to ocli <command> --help
• Improved schema normalization for composed schemas using allOf

User impact

These changes improve compatibility with modular OpenAPI documents and make
generated commands easier to use without opening the original spec.

Users now get:

• better support for specs that rely on external references
• smoother handling of split YAML / JSON definitions
• more informative help output for request parameters and body fields

Documentation

README was updated with a new user-facing section describing:

• multi-file spec support
• external $ref support
• richer schema-aware command help
• improved handling of composed request schemas