Show remote-but-online agents in @-mention autocomplete

[JRStriestocode]

Problem

When running a multi-machine community (e.g. a Mac Mini as an always-on agent host + a MacBook as the daily driver), agents created and running on machine A are invisible in the @-mention autocomplete on machine B — even though they are active channel members responding to mentions on the relay.

PR #1611 intentionally hid "non-invocable" agents from autocomplete to prevent silent mention failures to offline agents. But this filter treats remote-but-online agents the same as offline agents, making multi-machine setups unusable from the UI.

Expected behavior

The @-mention autocomplete should show all channel-member agents that are currently online on the relay, regardless of whether they are locally invocable by the current Desktop instance. An agent running on another machine in the same community and connected to the same relay is reachable — the relay delivers the mention, and the remote machine's buzz-acp executes it.

Current behavior

Autocomplete only shows agents registered in the local Desktop's agent list. Remote agents that are online channel members are hidden. The only way to mention them is through the CLI with an explicit --mention <pubkey>, which isn't accessible to normal users.

Suggested approach

• Use relay presence/online status to determine visibility, not local invocability
• Keep hiding agents that are both non-local and offline (the original intent of Hide non-invocable member agents from @-mention autocomplete #1611)
• Optionally show a visual indicator (e.g. a remote icon) to distinguish local vs remote agents in the picker

Workarounds tried

• Typing the name manually without autocomplete — does not work, mentions require autocomplete selection
• Using the CLI with --mention <pubkey> — works but not viable for day-to-day use
• Having another agent relay mentions — works but adds friction

Environment

• Buzz Desktop v0.5.x
• Two machines on the same relay/community
• Agents running via buzz-acp on the remote machine, confirmed online and responding to mentions sent via CLI

Related: #3277, #2508, #2349