[agent] feat: relay verified identity pairing

[alexng353]

[agent] Relay identity pairing through expiring opaque Noise messages, while clients verify the receiving terminal before releasing any encrypted identity.

• Enforce atomic single-receiver claims, source ownership, hashed receiver credentials, phase order, bounded payloads and ten-minute expiry. Delete relay contents on acknowledgement/cancellation and sweep expired rows.
• Bound active sessions by account, client IPv4/IPv6 network and total storage so creating more accounts cannot bypass the client quota.
• Adds an additive migration. All 40 API tests pass, including owner/token/phase checks, competing claims, expiration, cleanup and cross-account quota bypass. Real CLI pairing decrypts an existing project secret; rejected/cancelled transfers install nothing.

Deploy before the CLI 2.16.0 release. Existing invite routes are unchanged.

Co-Authored-By: GPT-6 (OpenAI) noreply@openai.com