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/hooks/agent-handoff/hook.ts

@@ -0,0 +1,37 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import { consumePendingHandoff } from "../../features/agent-handoff"
+import { log } from "../../shared/logger"
+
+const HOOK_NAME = "agent-handoff" as const
+
+export function createAgentHandoffHook(ctx: PluginInput) {
+  return {
+    event: async (input: { event: { type: string; properties?: Record<string, unknown> } }): Promise<void> => {
+      if (input.event.type !== "session.idle") return
+
+      const props = input.event.properties as Record<string, unknown> | undefined
+      const sessionID = props?.sessionID as string | undefined
+      if (!sessionID) return
+
+      const handoff = consumePendingHandoff(sessionID)
+      if (!handoff) return
+
+      log(`[${HOOK_NAME}] Executing handoff to ${handoff.agent}`, { sessionID })
+
+      try {
+        await ctx.client.session.promptAsync({
+          path: { id: sessionID },
+          body: {
+            agent: handoff.agent,
+            parts: [{ type: "text", text: handoff.context }],
+          },
+          query: { directory: ctx.directory },
+        })
+
+        log(`[${HOOK_NAME}] Handoff to ${handoff.agent} complete`, { sessionID })
+      } catch (err) {
+        log(`[${HOOK_NAME}] Handoff failed`, { sessionID, error: String(err) })
+      }
+    },
+  }
+}

src/hooks/agent-handoff/index.ts

@@ -0,0 +1 @@
+export { createAgentHandoffHook } from "./hook"

src/hooks/index.ts

@@ -56,3 +56,4 @@ export { createReadImageResizerHook } from "./read-image-resizer"
 export { createTodoDescriptionOverrideHook } from "./todo-description-override"
 export { createWebFetchRedirectGuardHook } from "./webfetch-redirect-guard"
 export { createLegacyPluginToastHook } from "./legacy-plugin-toast"
+export { createAgentHandoffHook } from "./agent-handoff";