Persist ERC-8004 agent identity

[bussyjd]

Summary

• Add a canonical x402/default AgentIdentity CRD that stores per-chain ERC-8004 registrations in status.registrations[].
• Move registration document publishing to identity-level active/tombstone rendering so the well-known document survives deletion of the last ServiceOffer.
• Make obol sell register idempotent per chain and add obol sell identity import for verified legacy agent IDs.
• Remove exposed custom identity refs and obsolete offer-coupled registration document builders.

Validation

• go test ./internal/erc8004 -count=1
• go test ./internal/serviceoffercontroller -count=1
• go test ./cmd/obol -count=1
• go test ./internal/embed -count=1
• go test ./internal/monetizeapi -count=1
• git diff --check

[bussyjd]

Superseded by #492 — folded into integration/post-490-cleanups as merge commit 8fb42c9 (Persist ERC-8004 agent identity). Closing to keep the open-PR list tidy. Re-open if anything was missed in the fold-in.