feat(athena): add session handoff with Question tool for Atlas/Prometheus routing

After Athena synthesizes council findings, presents user with Question tool
TUI to choose: Atlas (fix now), Prometheus (create plan), or no action.
On selection, session_handoff tool stores intent + calls updateSessionAgent(),
then agent-handoff hook fires on session.idle to switch the main session's
active agent via promptAsync with synthesis context.

src/config/schema/hooks.ts

@@ -55,6 +55,7 @@ export const HookNameSchema = z.enum([
   "todo-description-override",
   "webfetch-redirect-guard",
   "legacy-plugin-toast",
+  "agent-handoff",
 ])
 
 export type HookName = z.infer<typeof HookNameSchema>