Add buffered code-mode exec yields

[copyberry]

Add buffered code-mode exec yields

What changed

• Add the experimental code_mode_buffered_exec feature.
• When enabled, default code-mode exec calls without an explicit
  yield_time_ms to 30 seconds instead of 10 seconds.
• Report the effective default in the model-visible exec tool description.

Testing

• Verify that the generated tool description advertises the 30-second default
  when the feature is enabled.