docs: clarify artifact origin wording in README

[DigiEmu]

This PR performs a minimal README wording cleanup after the Core 2.0 Draft 4 Self-Audit release.

It replaces identity/trust wording that could imply DigiEmu Core certifies agent identity or trustworthiness:

• “Trusted identity layer” → “Artifact origin metadata”
• “Identity System” → “Artifact origin / bundle signer metadata”
• “Identity trust layer” → “Artifact origin / signer metadata utilities”
• “identity-bound artifacts” → “origin-bound or externally attested artifacts”

No code changes.

Checks:

• go test ./... passed
• go vet ./... passed