🧹 Refactor Long Function _evaluate_pull_request

[joy7758]

🎯 What: The code health issue addressed was the "Long Function" _evaluate_pull_request in src/titmas_action_gate/pr_gate.py.
💡 Why: Splitting this function into _validate_workspace and _generate_receipt reduces its cognitive load and improves readability. The original function was over 350 lines long.
✅ Verification: Verified changes via git diff. Formatted with ruff. Executed unit test suite PYTHONPATH=.:src uv run python3 -m unittest discover -s tests -v, and governance/milestone validation scripts uv run python3 scripts/validate_governance.py, uv run python3 scripts/validate_milestone.py which pass correctly.
✨ Result: Improved maintainability and modularity of the PR evaluation code flow without breaking any existing tests or integrations.

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PR created automatically by Jules for task 6606502389130054753 started by @joy7758

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