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.blocking.test.ts

@@ -79,7 +79,7 @@ describe("createBackgroundOutput block=true polling", () => {
     expect(output).not.toContain("Timed out waiting")
   })
 
-  test("returns latest output with timeout note when task stays running", async () => {
+  test("returns legacy status output with timeout note when task stays running", async () => {
     // #given
     let pollCount = 0
     const task = createTask({ status: "running" })
@@ -105,7 +105,7 @@ describe("createBackgroundOutput block=true polling", () => {
 
     // #then
     expect(pollCount).toBeGreaterThanOrEqual(2)
-    expect(output).toContain("# Full Session Output")
+    expect(output).toContain("# Task Status")
     expect(output).toContain("Timed out waiting")
     expect(output).toContain("still running")
   })