fix(athena): resolve task output files to project directory instead of process.cwd()

task-output-writer wrote to process.cwd()/.sisyphus/task-outputs/ while
council_finalize read from ctx.directory/.sisyphus/task-outputs/. When
these differ, all output files appear missing.

Fix by passing directory to writeTaskOutput from BackgroundManager, and
adding basePath to council_read. Also removes process.chdir() from tests
that was masking the bug.

src/plugin/tool-registry.ts

@@ -282,7 +282,7 @@ export function createToolRegistry(args: {
     ...hashlineToolsRecord,
     prepare_council_prompt: createPrepareCouncilPromptTool(ctx.directory),
     council_finalize: createCouncilFinalize(ctx.directory),
-    council_read: createCouncilRead(),
+    council_read: createCouncilRead(ctx.directory),
   }
 
   for (const toolDefinition of Object.values(allTools)) {