feat(node): Attest RPC + managed-state config (FM enrollment)

[Bugs5382]

What and why

Phase 2 (engine) of FM enrollment slice B (manager #21), consuming the merged api contract (api #46).

• Attest RPC — the node signs a manager-supplied nonce with its CA identity key (ceremony RootSigner; TPM-resident on TPM nodes, software on nodeID) over a SHA-384 digest, and returns the ASN.1 signature + the identity public key (PKIX/DER). Admin-authorized; Unimplemented in maintenance mode; empty nonce rejected. This is the challenge-response the Fleet Manager verifies and TOFU-pins. The production attester reuses the same node.KeyLoader→RootKeyBackend.LoadKey path as issuance/rekey/revoke and closes the signer per call. ek_pub/ek_cert reserved for v1.1 TPM-EK attestation (unset).
• Managed-state config — Config.Management { manager_cn, trust_pem, operator_surface_readonly }, carried symmetrically through ToProto/FromProto and validated, so a LINK enrollment can mark a node FM-managed via ApplyConfig (effective on reboot).

No EK/AK primitives (v1.1). Merge order: api (done) → this → manager → web.

Closes #167

Verification

• task ci (gofmt/vet/golangci-lint/test/build) green
• task license 0 flagged
• Attest handler tests (nil→Unimplemented, empty→InvalidArgument, unauthorized→PermissionDenied, delegate) + real ECDSA-P384 sign/verify round-trip
• Config round-trip tests (populated survives ToProto→FromProto; nil stays nil) + Validate

How this was verified

Security-focused opus review of Attest (signing correctness, no key leakage, authz-before-key, maintenance-nil) + a whole-branch opus review (config round-trip symmetric/proven, Attester wired only on the running node, no regressions across NodeService). Live attestation verify lands in Phase 3 (manager) + the pre-GOLD validation.