Add Proof Loop v0.3 adoption kit

[LeoStehlik]

Summary

• add a near-top README adoption path for using Proof Loop in a real repo
• add docs/adoption-kit.md with copy-paste builder/verifier flow
• add a compact completed checkout proof artifact under examples/adoption-kit
• bump skill metadata to v0.3.0 and include proof artifacts

Verification

• make test
• git diff --check
• bin/proof-loop check examples/adoption-kit/.agent/tasks/checkout-empty-state-proof
• bin/proof-loop validate examples/adoption-kit/.agent/tasks/checkout-empty-state-proof --require-evidence-json