🗺️ Atlas: Auto-generate environment variable documentation from source

[ToolchainLab]

💡 What: Auto-generate environment variable documentation from source.
🎯 Why: Prevents documentation drift by embedding config descriptions inside Settings using Pydantic Field.
🧪 Verification: Run uv run scripts/check_doc_env.py --sync locally.
🔒 Drift Prevention: Added scripts/check_doc_env.py which fails CI if README.md or .env.example drift from Pydantic config.
🗺️ Evidence: src/h4ckath0n/config.py is the single source of truth for config, scripts/check_doc_env.py generates the markdown section and .env.example.

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PR created automatically by Jules for task 1245481799607950163 started by @ToolchainLab

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