fix(core): keep required p tag on self-owned engrams

[slusset]

Summary

engram::build_event silently produced NIP-AE-invalid envelopes whenever an
identity keeps memory for itself (agent == owner): the required owner p
tag was missing from the signed event.

Root cause: nostr's EventBuilder strips self-referencing p tags by
default (an anti-self-notification convention). NIP-AE's envelope rules
require exactly one p tag naming the owner (docs/nips/NIP-AE.md,
Event envelope), and head selection queries filter on ["p", pubkey_o] —
so a self-owned write was accepted by relays but invisible to every read:
buzz mem set reported success while buzz mem get/ls returned nothing.

Fix

Call .allow_self_tagging() in build_event. One line, plus a regression
test pinning the self-owned round trip (envelope carries exactly one p tag
naming the owner; validate_and_decrypt succeeds).

Distinct-key behavior is unaffected — the NIP-AE reference test vectors and
the full engram suite pass unchanged (35 tests).

How this was found

Exercising NIP-AE from a single-identity deployment (a personal relay node
whose owner keeps engrams under its own key, rather than the hosted
agent-serves-owner pairing). NIP-AE's shapes work well for that case — the
spec doesn't forbid pubkey_a == pubkey_o, and the NIP-44 conversation key
is well-defined for self — this envelope bug was the only obstacle. If
maintainers would prefer the spec to state explicitly that self-owned pairs
are supported, happy to follow up with a one-line clarification to
docs/nips/NIP-AE.md.

Testing

• cargo test -p buzz-core engram — 35 passed, including the new
  self_owned_engram_keeps_required_p_tag regression test
• Verified live: self-owned buzz mem set/get/ls round-trips against a
  relay after the fix; before it, set succeeded and get returned
  not-found

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