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/agents/builtin-agents/general-agents.ts

@@ -11,7 +11,7 @@ import { applyModelResolution, getFirstFallbackModel } from "./model-resolution"
 import { log } from "../../shared/logger"
 
 export function collectPendingBuiltinAgents(input: {
-  agentSources: Record<BuiltinAgentName, import("../agent-builder").AgentSource>
+  agentSources: Partial<Record<BuiltinAgentName, import("../agent-builder").AgentSource>>
   agentMetadata: Partial<Record<BuiltinAgentName, AgentPromptMetadata>>
   disabledAgents: string[]
   agentOverrides: AgentOverrides