fix(core): Check the project of an existing system agent thread (no-changelog)

createThread checks access on the requested working project. It now
returns an existing thread only when that thread is in the same project.
A thread in another project is not found.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/cli/src/modules/agents/system-agents/__tests__/system-agent-execution.service.test.ts

@@ -1029,6 +1029,37 @@ describe('SystemAgentExecutionService', () => {
 			});
 		});
 
+		it('refuses to return an existing session of the user from another project', async () => {
+			const { service, threadRepository, txRunner } = setup();
+			threadRepository.findOneBy.mockResolvedValue({ ...thread, projectId: 'project-2' } as never);
+
+			await expect(
+				service.createThread({
+					agentId: AGENT_ID,
+					user,
+					projectId: 'project-1',
+					threadId: 'thread-1',
+				}),
+			).rejects.toThrow(NotFoundError);
+			expect(txRunner.run).not.toHaveBeenCalled();
+		});
+
+		it('returns an existing session of the user in the same project', async () => {
+			const { service, threadRepository, txRunner } = setup();
+			const existing = { ...thread, projectId: 'project-1' };
+			threadRepository.findOneBy.mockResolvedValue(existing as never);
+
+			expect(
+				await service.createThread({
+					agentId: AGENT_ID,
+					user,
+					projectId: 'project-1',
+					threadId: 'thread-1',
+				}),
+			).toBe(existing);
+			expect(txRunner.run).not.toHaveBeenCalled();
+		});
+
 		it('refuses to create a session with the id of a thread of another user', async () => {
 			const { service, threadRepository, txRunner } = setup();
 			threadRepository.findOneBy.mockResolvedValue({ ...thread, ownerId: 'user-2' } as never);

packages/cli/src/modules/agents/system-agents/system-agent-execution.service.ts

@@ -148,7 +148,9 @@ export class SystemAgentExecutionService {
 			if (
 				existing.agentId !== params.agentId ||
 				existing.accessScope !== 'user' ||
-				existing.ownerId !== params.user.id
+				existing.ownerId !== params.user.id ||
+				// The access check above ran for `params.projectId`, not for the project of this thread.
+				existing.projectId !== params.projectId
 			) {
 				throw new NotFoundError('Session not found');
 			}