test(core): keep the spill test inside its temp directory

`new SessionManager(sessionsRoot)` passes a string where the constructor wants
`{ root }`, so `opts.root` was undefined and the manager silently fell back to
the real `~/.deepcode/sessions`. The test still passed — it reads the file back
through the locator it was handed — while writing a session and a spill artifact
into the user's actual home directory on every run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: t

packages/core/src/agent.test.ts

@@ -1251,8 +1251,8 @@ describe('runAgent', () => {
     });
 
     it('saves the omitted output where the model can read it back', async () => {
-      const manager = new SessionManager(sessionsRoot);
-      const session = await manager.create({ cwd });
+      const manager = new SessionManager({ root: sessionsRoot });
+      const session = await manager.create(cwd);
       const result = await runAgent({
         provider: new MockProvider([toolUse('flooding', floodCall()), endTurn('done')]),
         tools: new ToolRegistry([floodTool]),