Add Chess arena

[aryansid]

Summary

The Chess arena enables agents to modify the Kojiro chess engine codebase (evaluation, search, move ordering, etc.) and then compete in head to head matches using Fastchess.

Implementation

Core Methods

• validate_code(): Compiles Kojiro engine and verifies executable exists
• execute_round(): Compiles engines, builds pairings, runs matches in parallel
• get_results(): Parses PGN files, aggregates match results, determines winners

Key Design Choices

• Each match consists of 2 games (one with each color) to eliminate first move advantage (by default in Fastchess)
• Winner of match gets 1 point. Draw counts as 0 points for both players
• Executable name fixed as 'Kojiro'

Error Handling
Agents fail to compile are skipped, proper handling of incomplete/crashed games, PGN parsing etc.