buzz-acp drops non-owner kind:9 mentions despite --respond-to anyone

## Bug Summary

`buzz-acp` with `--respond-to anyone` does not forward kind:9 mention events from non-owner channel members to the ACP agent. Only mentions from the channel owner are delivered.

## Reproduction

1. Community relay: `wss://sportstech.communities.buzz.xyz`
2. Channel: Group DM (4) `56e18629-d8f8-425d-9b4c-2c88082c081b`
3. Members: owner `2a68de51...`, agent `cbe410b8...` (Hermes Ali), user `34b9ee8e...` (SanthanaBharathy)
4. Start harness:
```bash
/Applications/Buzz.app/Contents/MacOS/buzz-acp --relay-url wss://sportstech.communities.buzz.xyz --private-key --agent-owner 2a68de51a9e98c1fd5fa2309330b1f7eb38dd6833c50c17758abe0dc4b8d7842 --agent-command /Users/ailab/.local/bin/hermes-acp --agents 10 --subscribe mentions --dedup queue --multiple-event-handling steer --respond-to anyone --model openrouter:deepseek/deepseek-v4 --memory --relay-observer
```
5. Owner sends `@Hermes Ali test` → **works**, ACP gets Prompt event, agent replies.
6. Non-owner user sends `@Hermes Ali what are you afraid of ?` → **no Prompt event in harness log, no agent reply.**

The non-owner events ARE in the relay (verifiable via `buzz messages get --channel 56e18629... --limit 10`), correctly p-tagged to the agent pubkey, and the user is a confirmed channel member with role=member.

## Variants tested (all fail identically)

- `--respond-to anyone` without `--agent-owner`
- `--respond-to anyone` + `--agent-owner `
- `--respond-to allowlist` + `--respond-to-allowlist 34b9ee8e...`
- `--subscribe mentions` + `--no-mention-filter`

## Evidence

- Agent log (buzz-acp output) shows `subscribed to channel 56e18629...` + `presence set to online` — never shows `Prompt on session` for non-owner mentions.
- Owner mention produces immediate `Prompt on session` + agent turn.
- Both sender events are identical in structure (kind:9, h-tag = channel-id, p-tag = agent-pubkey).

This blocks community agents from serving non-owner users — which defeats the "shared agent" use case.

## Environment

- Buzz.app current (Aug 2026)
- macOS 26.5.2
- buzz-acp binary from /Applications/Buzz.app/Contents/MacOS/buzz-acp