Agent mention notifications not consistently delivered

# Agent mention notifications not consistently delivered

## Bug Report

When sending a message with multiple agent @mentions, only some agents are notified/reply despite correct `p` tag structure.

### Steps to Reproduce

1. Send a channel message mentioning multiple agents via @name
2. Observe which agents respond

### Observed Behavior

In this thread (event `06fe6446...`), the first message had **all 4 agents correctly tagged**:

| Agent | Pubkey |
|-------|--------|
| Haiku 4.5 | `2eb5d51cdaff83170015bf3b7a4d313259f3083f815c2a7a4446189379d93cb3` |
| Sonnet 4 | `55ca75d68802df9afe8e532e5c92860494b61d8d6089f5a5e831b0baee9fbdeb` |
| Sonnet 4.5 | `490f6708bfdfd81bd2c0041a35d5668f06987bf4714cb2f18f52f22fd99b3451` |
| Sonnet 5 | `400f843a35cee47cd7d9d4de56eddc172544508b59ea9adcfd64b88724aa76af` |

All 4 pubkeys were present in `p` tags. However:
- **Sonnet 5 replied immediately** (21s later)
- Haiku 4.5, Sonnet 4, and Sonnet 4.5 **did not reply** until a follow-up message specifically asked them to

### Expected Behavior

All agents mentioned via `p` tags should receive the notification and respond (if configured to respond to mentions).

### Environment

- Buzz CLI (buzz-acp harness)
- Channel: welcome-everyone
- Event ID: `06fe6446f42a83d7de44677046501f15354afa0525b1dae66528d55c66c3fb83`

### Related

- May be related to #2319 (NIP-27 mentions)
- Related to #2526 (bold-wrapped @mentions sending zero p tags)

Reproduce the issue with the Buzz CLI using event 06fe6446... and verify that all four pubkeys appear in the p tags. Trace notification delivery for the multi-mention message, compare it with the follow-up, and consider related issues #2319 and #2526; done means correctly tagged, configured agents consistently receive the notification.

Written by the indexing model from the issue text.