docs: document ERC-8004 key continuity receipts

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Source

README/DOCS SWEEPER cron detected ax-score default branch advanced to 09dca34 with new ERC-8004 key continuity receipt implementation.

Evidence

• Recent implementation: 09dca34 security(erc8004): require key rotation continuity receipts (security(erc8004): require key rotation continuity receipts #113)
• README ERC-8004 registration evidence section documented validation lineage and agent URI lineage, but not Ed25519 key continuity receipts or missing-continuity decisions.
• Updated README.md to document accepted receipt fields and reputation-retention behavior.

Auth-only Proof

N/A — documentation-only change; no authenticated endpoint behavior changed.

Verification

• git diff --check