Move off meta.json-schema.tools as the $schema dialect and $ref target #423

Context

Our canonical schemas (spec/1.3/schema.json, spec/1.4/schema.json) still declare:

"$schema": "https://meta.json-schema.tools/"

and reference it internally:

"JSONSchema": { "$ref": "https://meta.json-schema.tools" }

That domain serves a copy of the draft-07 meta-schema with title annotations added (for prettier generated typing names — see open-rpc/meta-schema#323 and open-rpc/meta-schema#324 for the history). It works, but it is an external dependency baked into the spec itself: separate org, separate infra, and we have already been bitten:

• @open-rpc/playground had to vendor the JSONSchema definition locally as a stopgap because of CORS issues with https://meta.json-schema.tools (see the open-rpc/tools changelog for @open-rpc/playground@1.1.2)
• openrpc-linter rewrites $schema to http://json-schema.org/draft-07/schema# when it encounters it
• execution-apis tooling strips the embedded identifiers so the definition behaves like a local subschema

Proposal

Long term, the spec should not depend on a third-party domain for its own dialect identifier. Options:

• Reference the canonical draft-07 meta-schema URL (http://json-schema.org/draft-07/schema#) instead — simplest, but loses the title annotations that give us nice generated type names
• Self-host the annotated JSONSchema meta-schema under our own domain (e.g. alongside the versioned schemas from Serve versioned meta-schemas on meta.open-rpc.org and deprecate the legacy root URL #422), so the $schema / $ref targets are infrastructure we control
• Inline the JSONSchema definition into our schemas so there is no remote $ref at all

Tasks

• Decide which option we want
• Update spec/1.3/schema.json and spec/1.4/schema.json ($schema + JSONSchema $ref)
• Check downstream consumers that special-case the URL and update/remove workarounds: schema-utils-js (registers it in AJV under that key), playground (local vendoring workaround), openrpc-linter (rewrites it), execution-apis (strips the identifiers)
• Account for this in the deprecation plan in Plan to deprecate this repo — consolidate schema + typings into @open-rpc/spec and @open-rpc/spec-types meta-schema#476

Related: #422 (versioned meta-schemas on meta.open-rpc.org).