Decide Draft 2020-12 output contract

[crmne]

Goal

Decide and implement the final output contract for Draft 2020-12 JSON Schema documents.

Today to_json_schema returns a RubyLLM/provider-oriented envelope:

{
  name: "PersonSchema",
  description: "...",
  schema: {
    type: "object",
    properties: { ... }
  }
}

A pure Draft 2020-12 JSON Schema document would look like:

{
  "$schema" => "https://json-schema.org/draft/2020-12/schema",
  "title" => "PersonSchema",
  "description" => "...",
  "type" => "object",
  "properties" => { ... }
}

Decisions to make

• Should to_json_schema become the pure JSON Schema output?
• Should we add a temporary migration method first, such as to_json_schema_document?
• Should the current RubyLLM envelope remain available under another method until the next major release?
• Should the current name concept map to JSON Schema title?
• When do we remove provider-only keys such as strict from this gem?

Acceptance criteria

• Pure output validates against the Draft 2020-12 meta-schema.
• Output uses JSON-compatible string keys.
• Provider-only keys are not included in the pure schema document.
• README documents the migration path.
• RubyLLM compatibility expectations are clear before release.