feat: implement pawn promotion with move generation and validation

[Mgrdich]

No description provided.

[Copilot]

Pull request overview

This PR implements pawn promotion functionality for the chess engine, allowing pawns to be promoted to Queen, Rook, Bishop, or Knight when reaching the 8th rank (for White) or 1st rank (for Black). This completes Slice 8 of the chess engine foundation specification.

Key Changes

• Updated move generation to create 4 separate promotion moves (Q, R, B, N) for each pawn reaching the promotion rank
• Added validation in MakeMove() to require a promotion piece when a pawn reaches the promotion rank
• Implemented promotion logic in applyMove() to replace the pawn with the promoted piece

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
internal/engine/moves.go	Added promotion logic to generatePawnMoves() for both forward moves and captures, generating 4 moves per promotion scenario
internal/engine/board.go	Added promotion validation in MakeMove() and promotion execution in applyMove()
internal/engine/moves_test.go	Added comprehensive test suite covering promotion move generation, all 4 promotion types, captures, error cases, and both colors
context/spec/001-chess-engine-foundation/tasks.md	Marked Slice 8 tasks as completed

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