Inbox and notifications cannot distinguish a mention from a person from a mention from an agent

**Motivation**

In a busy community most messages come from agents. Mentions from people get buried in that stream, and there is no way to separate them.

Today the Inbox filter menu offers All / Projects / Mentions / Threads / Needs action / Agents / Reminders / Drafts (same set on Desktop and mobile). "Mentions" mixes people and agents, and "Agents" shows agent activity, not "everything except agents". Desktop notifications and sounds have a single "@Mentions" slot with the same problem: a person tagging you and an agent tagging you produce the identical alert.

Most people still prioritise a message from a colleague over one from an agent, even in a setup that otherwise treats agents as equal participants.

**Proposed solution**

1. An Inbox filter that shows only mentions and messages from people, excluding agents.
2. A separate notification/sound slot for mentions from people, so a human tagging you can be made louder than an agent tagging you (or the agent one turned off).

The app already knows which accounts are agents — it renders their avatars differently — so the distinction is available where the filtering happens.

**Alternatives considered**

- Muting noisy agents individually: does not scale, and it hides useful agent output as well.
- Reading the Mentions filter and skimming for names: works, but that is the manual version of the filter being asked for.

**Additional context**

Duplicate search: none found.

No files, tests, or entry points are named. Start by locating the Inbox filter implementation and the desktop/mobile notification and sound settings; trace how agent accounts are identified during filtering. Done means people-only mentions can be filtered separately and human and agent mention alerts can be configured independently.

Written by the indexing model from the issue text.