Close the A2A delegation auto-resume gap.
Root cause: heartbeat.py's _check_delegations already writes completed
delegation rows to DELEGATION_RESULTS_FILE and sends a self-message to
wake the agent. executor_helpers.read_delegation_results() was defined to
atomically consume that file, but a2a_executor._core_execute() never
called it — so delegation results were written but the agent never saw
them.
Fix: call read_delegation_results() at the top of _core_execute() and
prepend the results to the user input context so the agent can act on
them without an explicit check_task_status call. The Temporal durable
workflow path is also covered because it calls _core_execute() directly.
Test: two new cases — delegation results injected when file exists;
user input passed through unchanged when file is empty.
Closes molecule-core#354.
