Define private managed agent wire protocol (#4593)

## Summary

- reserve kind `30179` for owner-private managed-agent aggregates
- define the fail-closed owner-self NIP-44 v2 envelope and versioned
payload codec
- bind runnable identity/configuration to complete signed
`30175`/`30177` recovery projections
- validate NIP-OA owner→agent attestations and reject self-attestation
- document NIP-PMA authority, migration prerequisites, privacy, and
deployment order
- keep generic relay ingest closed until private storage and atomic
aggregate CAS exist

## Safety boundary

This is the inert protocol/codec slice only. It does not publish
secrets, change agent authority, migrate local records, or enable kind
`30179` ingestion. The relay regression test proves generic EVENT ingest
still rejects the kind.

The finalized migration plan adds later prerequisites for relay-private
storage/CAS, runtime lease/fencing, Desktop cutover, and harness
authentication. Those belong in staged follow-up PRs rather than
expanding this inert foundation.

## Validation

At commit `67f0ea4ebb8d3ccba3a3eb9374e89a7178913f74`:

- `cargo test -p buzz-core` — 246 unit + 2 doc tests passed
- `cargo test -p buzz-relay
private_managed_agent_kind_remains_rejected_until_atomic_ingest_exists`
— passed
- push hooks: Rust tests and desktop checks passed (`2145` desktop tests
passed, `14` ignored)
- `cargo fmt --all -- --check`
- `git diff --check`

## Review

Princess Donut cleared security/data integrity with no remaining
high/medium findings. Mongo cleared migration compatibility and wire
grammar. The later runtime lease/fencing protocol was also adversarially
cleared as a plan; implementation slices still require independent
evidence before activation.

Deterministic plaintext/signed-projection/auth-tag interoperability
vectors remain a valuable follow-up, not an S0 merge gate; random NIP-44
ciphertext is intentionally not snapshotted.

---------

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>

crates/buzz-relay/src/handlers/ingest.rs

@@ -3216,6 +3216,18 @@ mod tests {
         }
     }
 
+    #[test]
+    fn private_managed_agent_kind_remains_rejected_until_atomic_ingest_exists() {
+        assert!(
+            required_scope_for_kind(
+                buzz_core::kind::KIND_PRIVATE_MANAGED_AGENT,
+                &make_dummy_event(),
+            )
+            .is_err(),
+            "kind 30179 must not enter generic EVENT ingest before privacy and aggregate CAS deploy"
+        );
+    }
+
     #[test]
     fn ephemeral_kinds_not_in_scope_allowlist() {
         assert!(required_scope_for_kind(KIND_PRESENCE_UPDATE, &make_dummy_event()).is_err());