Agent reply visible in Activity log but not displayed in channel/thread

## Summary

When an agent (e.g., Fizz, Bumble, Honey) is tagged in a channel message and responds, the reply is sometimes visible in the Activity/Pulse log but does **not** appear in the channel or create a new thread against the original message.

## Observed Behavior

1. User sends a message tagging an agent (e.g., "@Fizz hello") in a channel (e.g., #Welcome)
2. The agent processes the message and generates a reply
3. The reply appears in the **Activity** or **Pulse** log — confirming the agent completed its response
4. However, the reply is **not visible** in the channel
5. No new thread is created against the original message
6. The user has no indication in the conversation view that the agent responded

## Expected Behavior

When an agent replies to a message in a channel, the reply should:
- Appear in the channel as a visible message (or as a threaded reply to the original message)
- Be accessible from the conversation UI without switching to the Activity/Pulse view

## Environment

- **Buzz Desktop version:** 0.5.2 (bundle xyz.block.buzz.app)
- **OS:** macOS (arm64)
- **Agent runtime:** OpenCode 1.18.9-fork
- **Relay:** `wss://kamil22.communities.buzz.xyz`

## Notes

- This is **intermittent** — sometimes replies appear correctly, other times they only show in Activity
- Agent logs show successful initialization and message processing
- No errors are reported in the agent or buzz-acp logs when this occurs
- The issue may be related to how the relay routes agent responses back to the channel vs. storing them only as activity events

## Suggested Investigation

- Check if the agent response event includes the correct channel/thread routing metadata
- Verify that the relay is forwarding the response back to the channel subscribers (not just the Activity feed)
- Check if there's a race condition between the agent's response and the channel subscription state

Start by reproducing a tagged-agent reply in a channel and compare the Activity/Pulse event with the channel and thread state. Trace the relay response routing and channel subscriber delivery, including the suggested routing metadata and subscription race; done means the reply reliably appears in the channel or as a thread without opening Activity/Pulse.

Written by the indexing model from the issue text.