Fix code rule violations

[xenobiasoft]

Description

Fixes violations of the project's defined rules found via a codebase review against .claude/rules/ standards.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Other (please describe):

Changes Made

• Domain exception: Added CellAlreadyHasValueException and replaced two InvalidOperationException throws in Cell.AddPossibleValue and SudokuGame.AddPossibleValueToCell — domain layer must only throw domain-specific exceptions
• Dead error handling: Removed unreachable try/catch(Exception) in UpdateProfileAliasCommandHandler that wrapped only a LogDebug call (nothing inside could throw)
• Magic numbers: Extracted difficulty empty-cell count ranges to named private const int constants in PuzzleGenerator
• Comments: Removed "what" comments and a multi-line docstring from Cell that described what the code already made obvious
• Tests: Updated two tests asserting InvalidOperationException to assert CellAlreadyHasValueException

Testing

• I have tested these changes locally
• All existing tests pass
• I have added new tests (if applicable)

Screenshots (if applicable)

N/A

Checklist

• My code follows the project's coding standards
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have updated the documentation (if necessary)

[github-actions]

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