fix(athena): make council tool blocking — collect results directly instead of polling

The athena_council tool now waits for all council members to complete and
returns their collected results as markdown, eliminating the need for
Athena to repeatedly call background_output per member (which created
excessive UI noise).

- Add result-collector.ts that polls task status and fetches session content
- Update tool to accept BackgroundOutputClient and return formatted markdown
- Update Athena prompt to remove background_output polling steps
- Rewrite tests for new blocking behavior and markdown output format

src/plugin/tool-registry.ts

@@ -177,6 +177,7 @@ export function createToolRegistry(args: {
   const athenaCouncilTool = createAthenaCouncilTool({
     backgroundManager: managers.backgroundManager,
     councilConfig: athenaCouncilConfig,
+    client: ctx.client,
   })
 
   const isMultimodalLookerEnabled = !(pluginConfig.disabled_agents ?? []).some(