feat: added attempt_complete tool, giving agent ability to signal completion

[ErikBjare]

Played with this a couple of days ago, idea was to mimic a similar tool I saw in one of the SWE-Bench submissions.

Could be a more reliable stopping-condition (and a good place to run appropriate checks) than waiting for a non-tooluse message.

Not sure if this should be merged here or in gptme-contrib, but since it required some changes to the Python tool it'll go here for now.

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[!IMPORTANT] Add attempt_complete tool to signal task completion, integrated into Python execution flow in python.py.

• New Tool:
  • Added attempt_complete.py with Completed class to signal task completion.
  • attempt_complete() function attempts task completion and returns Completed.
  • check_completion() placeholder for task completion checks.
• Integration:
  • In python.py, import Completed from attempt_complete.py.
  • Update execute_python() to handle Completed by exiting process.
• Misc:
  • attempt_complete tool is disabled by default in ToolSpec.

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