Export EffortLevel type alias

[Jaksenc]

Summary

• add a public EffortLevel type alias for Claude effort string levels
• reuse the alias in AgentDefinition and ClaudeAgentOptions
• export the alias from the package root for downstream SDK wrappers

Closes #938

Testing

• .venv/bin/python -m pytest tests/test_types.py -q
• .venv/bin/python -m ruff check src/claude_agent_sdk/types.py src/claude_agent_sdk/__init__.py tests/test_types.py
• .venv/bin/python -m ruff format --check src/claude_agent_sdk/types.py src/claude_agent_sdk/__init__.py tests/test_types.py
• git diff --check

[codecov-commenter]

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Codecov Report

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Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage        ?   89.09%           
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  Files           ?       23           
  Lines           ?     3980           
  Branches        ?        0           
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  Hits            ?     3546           
  Misses          ?      434           
  Partials        ?        0           

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