fix(background-output): stop defaulting full_session=true for running tasks

background_output auto-enabled full_session when the task was still
running, returning the entire session transcript on every poll. When
the parent agent had no other work and polled in a tight loop, this
caused massive token waste because each response dumped thousands of
tokens into the conversation history.

Default full_session to false so running-task checks return a compact
status table (~200 tokens). Callers can still pass full_session=true
explicitly when they need the full transcript.

src/tools/background-task/create-background-output.ts

@@ -78,7 +78,7 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
         }
 
         const isActive = task.status === "pending" || task.status === "running"
-        const fullSession = args.full_session ?? isActive
+        const fullSession = args.full_session ?? false
         const includeThinking = isActive || (args.include_thinking ?? false)
         const includeToolResults = isActive || (args.include_tool_results ?? false)