feat(01-02): add athena to agent name contracts

- add athena to built-in and overridable agent name schemas

- extend BuiltinAgentName with athena for config-level recognition

- make builtin agent source maps partial until athena runtime registration lands

src/config/schema/agent-names.ts

@@ -12,6 +12,7 @@ export const BuiltinAgentNameSchema = z.enum([
   "momus",
   "atlas",
   "sisyphus-junior",
+  "athena",
 ])
 
 export const BuiltinSkillNameSchema = z.enum([
@@ -39,6 +40,7 @@ export const OverridableAgentNameSchema = z.enum([
   "explore",
   "multimodal-looker",
   "atlas",
+  "athena",
 ])
 
 export const AgentNameSchema = BuiltinAgentNameSchema