Remote ACP agents (channel role 'bot') do not appear in Desktop @mentions autocomplete dropdown

# Remote ACP agents (channel role 'bot') do not appear in Desktop @mentions autocomplete dropdown

### Summary

When an ACP agent is hosted remotely (e.g., running `buzz-acp` on a server or VPS) and joined to a community channel with `role: "bot"`, human developers using Buzz Desktop cannot find or autocomplete the agent when typing `@` in the channel chat.

While typing `@AgentName` manually sends the message, the UI dropdown never lists the remote agent. This causes confusion for team members, making remote server agents appear non-mentionable or offline.

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### Environment

- **Buzz Desktop:** Windows / macOS / Linux (latest release)
- **Harness:** `buzz-acp` running on a remote Linux server connecting to a community relay (`wss://...`)
- **Channel Membership:** Remote agent has `role: "bot"` in the channel.

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### Root Cause Analysis

Inspection of the Desktop UI client logic (`managed_agents`, `change_channel_member_role`, and mention suggestion filters) reveals the following split:

1. **Human Member Filter:** The `@` mention autocomplete generator queries channel members but filters out any identity where `role === "bot"`.
2. **Local Agent Filter:** The autocomplete generator also queries `managed_agents` / `custom_harnesses`, but this only includes agents that have a local harness configuration present on that individual developer's machine.
3. **The Remote Agent Gap:** A remote agent hosted 24/7 on a server has `role: "bot"` on the relay and has *no local harness* on the developer's machine. Consequently, it is excluded from **both** the human member list and the local agent list in the `@` autocomplete dropdown.
4. **Immutable Bot Role in GUI:** Additionally, the Buzz Desktop Channel Members modal disables the role-editing dropdown for identities with `role: "bot"`, so a channel owner cannot reassign the bot to `role: "member"` through the UI.

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### Steps to Reproduce

1. Deploy an ACP agent on a remote server running `buzz-acp` connected to a Buzz community relay.
2. Ensure the remote agent joins `#general` with `role: "bot"`.
3. Open Buzz Desktop on a developer machine (with no local agent configured for that identity).
4. Go to `#general` and type `@`.
5. Observe: Human members (e.g. `owner`, `admin`, `member`) appear in the suggestion dropdown, but the remote bot does not appear even when typing its full name.

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### Expected Behavior

Any channel member (including identities with `role: "bot"`) or remote community agent should appear in the `@` autocomplete dropdown (ideally with a `[Bot]` or `[Agent]` badge), allowing any developer to easily discover and mention them without needing a local harness.

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### Proposed Solutions

1. **Include `role: "bot"` in Mention Suggestions:** Update the channel mention suggestion filter in Desktop so that channel members with `role: "bot"` are included alongside human members.
2. **Allow Remote Agent Discovery:** Populate the mention list with all active identities in the channel member roster.
3. **Channel Role Management:** Enable community owners/admins to adjust member roles or manage bot visibility from the Desktop UI.

Start with the Desktop UI mention suggestion filters, including the managed_agents and custom_harnesses paths, and compare them with the channel member roster and change_channel_member_role logic. Reproduce with a remote buzz-acp identity whose channel role is bot; done means that identity appears in @ autocomplete without a local harness and the relevant behavior is covered by the existing UI checks.

Written by the indexing model from the issue text.