fix(tasks): fail loud when the abstract seam is mounted directly

Review finding (Codex round 1): abstract erases at runtime and
@deepseek-ai/dsh-tasks used to be the mountable registry, so a stale
composition row would register a ctx.tasks with no method implementations
and fail far from the misconfiguration. The seam constructor now rejects
direct mounts with a load-time pointer at dsh-tasks-local; the seam suite
pins the fence, the Agent Note cost paragraph records the actual behavior,
and the stale tool-pty README requirement line names the implementation
package.

packages/tasks/tasks/tests/service.spec.ts

@@ -79,4 +79,10 @@ describe('TaskService seam', () => {
     class SecondTaskService extends StubTaskService {}
     await expect(ctx.plugin(SecondTaskService)).rejects.toThrow(/service "tasks" has been registered/)
   })
+
+  it('mounting the abstract seam directly fails loudly at load (stale-composition fence)', async () => {
+    const ctx = new Context()
+    await expect(ctx.plugin(TaskService as unknown as typeof StubTaskService))
+      .rejects.toThrow(/abstract task registry seam; load an implementation such as @deepseek-ai\/dsh-tasks-local/)
+  })
 })