engine: Stop works in every state, not just between iterations

Stream drops between chunks (+ the pre-first-token wait); pending approvals/questions/plans resolve as interrupted.
Running shell commands die via an executor interrupt hook; skipped tool calls still get results (no orphans).
Also fixes delete_session calling a nonexistent engine.interrupt().

coworker/agent.py

@@ -308,6 +308,8 @@ def build_engine(
         model=model,
         instructions=instructions,
         approver=approver,
+        # Stop kills the in-flight foreground shell command, not just the loop.
+        interrupt_hooks=[executor.interrupt_now] if executor is not None else None,
         max_iterations=(
             max_iterations if max_iterations is not None else config.max_iterations
         ),