refactor(schema): migrate from invopop/jsonschema to google/jsonschema-go

[tak848]

Why

We used invopop/jsonschema only to generate JSON Schema from Go types — the editor config schemas (schemas/*.json) and the LLM structured-output schema. Its Schema.Properties is an ordered-map whose backing package invopop swaps between releases (wk8/go-ordered-map → pb33f/ordered-map in v0.14), which broke our build because config.go imported that package directly.

What

Migrate all four schema-generation sites to google/jsonschema-go, which is already in the dependency graph (via anthropic-sdk-go → modelcontextprotocol/go-sdk) and models property order with a plain []string field instead of an ordered-map dependency. Our code can no longer be broken by an ordered-map swap. invopop / pb33f stay in the build solely as anthropic-sdk-go transitive deps; we no longer import them directly.

• config.go: build the provider.auth oneOf with google's Schema API, exported as AuthConfigSchema() (google has no custom-schema interface, so the generator wires it in explicitly).
• genschema: reflect via jsonschema.For[config.Config]; collapse google's nullable spelling ("type": ["null", T], emitted for Go pointers/slices) back to the bare type so the editor schema stays strict; overwrite provider.auth to avoid the pointer-null widening making the oneOf unsatisfiable.
• LLM clients: build the output schema via jsonschema.For[llm.Output]; output is object + additionalProperties:false + all-required, matching OpenAI strict mode and Anthropic tool schema. llm.Output struct tags: jsonschema_description → jsonschema.

Schema diff

Regenerated schemas/*.json are semantically identical (verified by deep key-sorted comparison) except provider.auth now carries the description the code always intended — invopop silently dropped it. Everything else is JSON key-order only.

Verification

• go build ./..., go vet ./..., go test -race -cover ./... all pass.
• No direct imports of invopop/jsonschema, pb33f/ordered-map, or wk8/go-ordered-map remain.
• Deep-sorted schema comparison confirms no validation-behavior change.
• LLM output schema is OpenAI strict-compatible (object, additionalProperties:false, all keys required, no $schema).