v0.8.0: P-256 by default; claim status adds registeredAt and opt-in ownerEmail

Breaking: register() and generateKeyPair() create a P-256 (ES256) key by default, the key type AP2 receipts need. Pass keyAlgorithm: 'Ed25519' for the old default; the broker accepts both. To move an existing Ed25519 agent to P-256, register a new agent.

• getClaimStatus() returns registeredAt, and ownerEmail / ownerEmailVerified only when the person who claimed the agent chose to share their email with your platform.
• README: claiming an agent for an organization; opt-in owner email; the public registry's minimal record for revoked agents; what an AP2 mandate must say when you're the merchant (merchant/payee, aud, nonce), P-256 for AP2 receipts, and fresh root-only mandates. The verifyMandate() example now passes your DID as expectedAudience.