fix(background-task): restore opt-in full session output

Bring background_output back to the legacy contract so callers only get full session transcripts when they explicitly ask for them.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -53,7 +53,7 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
           "Wait for completion (default: false). System notifies when done, so blocking is rarely needed."
         ),
       timeout: tool.schema.number().optional().describe("Max wait time in ms (default: 60000, max: 600000)"),
-      full_session: tool.schema.boolean().optional().describe("Return full session messages with filters (default: true)"),
+      full_session: tool.schema.boolean().optional().describe("Return full session messages with filters (default: false)"),
       include_thinking: tool.schema.boolean().optional().describe("Include thinking/reasoning parts in full_session output (default: false)"),
       message_limit: tool.schema.number().optional().describe("Max messages to return (capped at 100)"),
       since_message_id: tool.schema.string().optional().describe("Return messages after this message ID (exclusive)"),
@@ -122,10 +122,7 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
         }
 
         const isActive = isTaskActiveStatus(resolvedTask.status)
-        const fullSessionProvided = args.full_session !== undefined
-        const fullSession = fullSessionProvided
-          ? (args.full_session ?? true)
-          : true
+        const fullSession = args.full_session ?? false
         const includeThinking = isActive || (args.include_thinking ?? false)
         const includeToolResults = isActive || (args.include_tool_results ?? false)