Add stage_hunk interactive staging tool

[GuyPaddock]

Summary

• introduce a new stage_hunk tool with schema support and wire it into the CodeAct agent tooling stack
• add runtime handling and git utilities to expose selectable unstaged hunks and apply staged subsets to the index
• extend schemas, serialization, and unit coverage for the new staging workflow

Testing

• python -m pytest tests/unit/agenthub/test_function_calling.py::test_stage_hunk_with_lines tests/unit/agenthub/test_agents.py::test_stage_hunk_tool tests/unit/events/test_action_serialization.py::test_stage_hunk_action_serialization_deserialization (fails: missing litellm dependency in environment)

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