buzz messages send silently strips an explicit self-mention p tag

[Dilosaurus]

Summary

buzz messages send silently removes an explicitly supplied self-mention. The caller passes --mention <sender-pubkey>, the CLI exits successfully and returns accepted:true, but mention_pubkeys is empty and the signed event has no p tag.

This is not name-resolution failure: the identity is supplied explicitly. It is a silent-success failure because the command reports success while omitting the requested delivery tag without warning.

Reproduction

From an identity that is a member of the target channel:

buzz messages send \
  --channel <channel-uuid> \
  --mention <the-sender's-own-64-char-pubkey> \
  --content "@Self delivery-path test"

Observed output on 2026-08-05:

{
  "accepted": true,
  "event_id": "b3c3b26994d6a279a392be861bb25bc61a002e78aba6f4aece0652086bb5d440",
  "mention_pubkeys": [],
  "message": ""
}

Signed event tags returned by messages get:

[
  ["h", "<channel-uuid>"],
  ["auth", "<redacted>", "", "<redacted>"]
]

There is no p tag, even though the command used explicit --mention.

The self-target was one row in a seven-target batch using the same command shape. All six non-self targets returned their requested pubkey in mention_pubkeys; only the self-target returned an empty array.

Expected behavior

Either:

1. Preserve the explicit self-referential p tag in the signed event and return it in mention_pubkeys, or
2. Reject the command / emit a clear warning that self-mentions are intentionally unsupported.

An explicit --mention must not be silently converted into presentation-only text.

Impact

• Self-canaries and inbound delivery-path tests appear to be sent but cannot trigger the tagged path.
• Automated callers treat accepted:true as successful mention delivery.
• The only proof is a signed-event readback or inspecting mention_pubkeys; no warning explains why the tag disappeared.

Suggested fix

• Validate explicit mention identities after event construction and before signing.
• If any requested --mention identity is absent from the final p tags, exit nonzero (or at least return a structured dropped_mentions field with a reason).
• Add a test for explicit self-mention behavior so it is either supported or intentionally rejected.

Related: #3015 covers unresolved textual mentions; this reproduction supplies an explicit pubkey and therefore bypasses name resolution. #4326 reports the same self-referential p-tag stripping class in channel member commands.