Adding a schema codec

A schema codec speaks one version of the System One API. Adding one, for example a new release of the standard as systemone/0.3, touches no caller: configuration resolves schema through the registry, and the Router's client and listener paths go through the codec interfaces.

1. Describe the specification. Add its file under config/schemas/systemone and list it in that directory's README. For the standard, write our own description of the interface (field names, types and structure, in our own words), with source and the source_sha256 you checked it against; never copy a document that carries no license. Run make systemone-schemas-drift to compare the description with the published document.
2. Regenerate the tables. make systemone-schemas-generate records the file and its sha256 in pkg/systemonecodec/zz_generated_specs.go; the registry refuses a codec whose specification does not match.
3. Write the codec in src/semantic-router/pkg/systemonecodec: a stateless type with ID, Spec, Capabilities, Plan, DecodeReply, ListingPath and DecodeListing. Plan decides every wire call of a batch, one batch call or one call per request; read and write only the IR (systemoneprotocol), never another codec's wire form. A schema a listener may serve also gets a ListenerCodec (see listener.go).
4. Register it in Builtin() (registry.go) and add its ID to the systemone entry of pkg/apiregistry (upstream schemas, and ListenerSchemas when a listener may serve it).
5. Test it: golden plans and replies, the refusals at load, its errors and listing, and the conformance tests against a mock server built from the specification.
6. Document it: the System One schemas page and the compatibility matrix, which is generated from the specifications.