Extract ExecutionHistory from TransitionContext (Issue #20)

[benrowe]

Summary

Extracted execution history tracking from TransitionContext into a dedicated ExecutionHistory class to improve separation of concerns and follow the Single Responsibility Principle.

Changes:

• Created new ExecutionHistory class with immutable recording pattern
• Refactored TransitionContext to delegate execution tracking to ExecutionHistory
• Updated serialization/unserialization to work with new structure
• Maintained 100% backward compatibility - all existing tests pass without modification

Key Design Decisions:

• Immutable pattern: All recording methods return new instances (consistent with existing collections)
• Type-safe: Leverages existing typed collections (GateEvaluationCollection, ActionResultCollection, ActionSkipCollection)
• API preservation: TransitionContext public API remains unchanged
• Backward compatible serialization: JSON format identical for existing data

Test Coverage

• 35 new tests added for ExecutionHistory class
• All 328 tests passing (up from 287)
• 100% code coverage maintained (586 lines covered)
• Zero breaking changes - all existing tests pass

Benefits

✅ Better separation of concerns - ExecutionHistory handles tracking, TransitionContext handles orchestration
✅ Easier to test independently
✅ Improved maintainability with clear component boundaries
✅ Type-safe with immutable collections

Test Plan

• Run full test suite: make test (328 tests, 1125 assertions)
• Verify code coverage: make test-coverage (100% coverage)
• Run quality checks: make check (all green)
• Verify backward compatibility: All existing tests pass
• Verify serialization: All serialization tests pass

Closes #20

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 noreply@anthropic.com