fix(session): harden shell cancellation (#24553)

packages/opencode/test/effect/runner.test.ts

@@ -334,6 +334,22 @@ describe("Runner", () => {
     }),
   )
 
+  it.live(
+    "cancel does not mask shell defects",
+    Effect.gen(function* () {
+      const s = yield* Scope.Scope
+      const runner = Runner.make<string>(s, { onInterrupt: Effect.succeed("interrupted") })
+
+      const sh = yield* runner
+        .startShell(Effect.never.pipe(Effect.ensuring(Effect.die("boom")), Effect.as("ignored")))
+        .pipe(Effect.forkChild)
+      yield* Effect.sleep("10 millis")
+
+      yield* runner.cancel
+      expect(Exit.isFailure(yield* Fiber.await(sh))).toBe(true)
+    }),
+  )
+
   // --- shell→run handoff ---
 
   it.live(