feat: NMv3: announce agents' ed25519 identities to the contract

[jstuczyn]

Further builds up on the existing liveness tests stack. The agent derives an ed25519 client identity from its noise key and announces it, and the orchestrator records it on chain.

• Agent derives the identity from its x25519 noise private key via HKDF-SHA256 with a domain-separation label, using the 32-byte output directly as the ed25519 seed. No new key material on disk, no operator step. Pinned by a known-answer test, because changing the label would silently invalidate every identity already announced.
• Orchestrator carries the identity on POST /v1/agent/announce, holds it in the KnownAgents entry, and puts it on both AuthoriseNetworkMonitor messages of the existing single transaction. An announcement whose identity differs from the cached one resets the announced flag so both addresses are re-authorised.

This lands dark. The field is written but nothing reads it until the gateway session gate from branch 3 meets an agent that announces, which is branch 9.

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