feat(01-02): wire athena overrides into config validation

- add AthenaOverrideConfigSchema so athena supports council plus standard override fields

- export athena schema/contracts and add root config tests for valid and invalid athena overrides

- switch schema generation to zod v4 toJSONSchema and regenerate JSON schema with athena council structure

src/config/schema.ts

@@ -1,5 +1,6 @@
 export * from "./schema/agent-names"
 export * from "./schema/agent-overrides"
+export * from "./schema/athena"
 export * from "./schema/babysitting"
 export * from "./schema/background-task"
 export * from "./schema/browser-automation"