[Bug] Remote Agent Missing from @ Mention Autocomplete Across Machines

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> Issue generated by 5.6 Sol High
```

**Describe the bug**

An agent named `Atlas`, managed and running on Machine A, is not discoverable or mentionable from Machine B, even though:

- Both users are members of the same channel.
- The agent is a member of that channel.
- The agent’s **Respond-to** setting is **Anyone**.
- Both machines are connected to the same Buzz community/relay.

Machine B’s `@` autocomplete only shows agents managed locally by Machine B. Manually typing `@Atlas` does not trigger the remote agent because the message is not sent with the agent’s Nostr `p` mention tag.

**To Reproduce:**

1. On Machine A, create and start a managed agent named `Atlas`.
2. Set the agent’s **Respond-to** setting to **Anyone**.
3. Add `Atlas` and the Machine B user to the same channel.
4. Confirm on Machine A that `Atlas` is running and listed as a channel member.
5. Open the same channel on Machine B.
6. Type `@` in the message composer.
7. Observe that `Atlas` is missing from autocomplete.
8. Manually type `@Atlas` and send the message.
9. Observe that the agent does not respond.

**Expected behavior:**

A running agent that:

- is a member of the current channel, and
- has **Respond-to: Anyone**

should appear in the `@` autocomplete for users on other machines. Selecting it should add the agent’s pubkey as a Nostr `p` tag so the remote agent receives and responds to the message.

**Supporting Material**

Screenshots show:

- The remote agent running on Machine A.
- **Respond-to** configured as **Anyone**.
- The remote agent listed as a member of the Test channel.
- Machine B’s autocomplete showing only locally managed agents and omitting the remote agent.
- Manually typed `@Atlas` messages receiving no response.
- The agent responding successfully when invoked from Machine A.

Technical observation: the client initially recognizes shared relay agents with `respond_to: anyone` as mentionable, but a subsequent filter removes every agent that is not present in the

Start with the message composer’s @ autocomplete and the subsequent filter for shared relay agents described in the issue. Trace how selecting or manually entering an agent becomes a Nostr p mention tag, then verify that eligible remote agents appear and receive messages while local-agent behavior remains unchanged.

Written by the indexing model from the issue text.