Add player artifact upload skill and contract docs

[aaln]

Adds the upload-player-artifact skill plus the PLAYER_ARTIFACT.md contract docs, ported from the metta monorepo.

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

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🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
33755224	Triggered	Generic High Entropy Secret	0304826	pyproject.toml	View secret

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