feat(desktop): show each agent's role/description in the @-mention selector

## Problem

The `@`-mention selector lists agents by **display name + avatar only** — there's nothing to tell you what each agent is *for*. In a workspace with several agents (especially a themed set that share a visual style), they're near-indistinguishable at the point of use. You have to already know each agent's role from memory, or scroll back to whenever they introduced themselves.

Concrete case: a hive with three bee agents — Fizz (builder), Honey (writer), Bumble (researcher) — renders as three near-identical rows in the selector. A user hovering the composer can't tell who to call for "debug this" vs "summarize this thread" vs "go research X." Their words: *"we have 3 bee agents, but it's so hard to tell which one does what. Whom should I call for when I need to do something?"*


## Proposed UX

- In the `@`-mention autocomplete row, show a **one-line role/description** under (or beside) each agent's display name — same pattern the slash-command autocomplete in #2528 proposes for command descriptions.
- Source the line from the agent's public kind:0 profile `about` field. Fall back to nothing (name only, as today) when `about` is empty, so this degrades gracefully.
- Keep it to one line, truncated — the selector is a quick picker, not a profile card.

## Data-source note

- Agents don't reliably publish an `about` today (observed kind:0 profiles carry only `display_name` + `picture`). Surfacing the field is only useful if agent creation seeds a short role string into `about`. Worth pairing this with a default: when an owner drafts an agent, derive a one-line `about` from its purpose/system-prompt.
- Related: #2534 — kind:0 profile republish drops every field outside a 5-key allowlist. If `about` isn't in that allowlist, it'll get silently stripped on republish and this feature will show blanks. That allowlist needs `about` preserved for this to hold.

## Non-goals

- Full agent profile cards / hover popovers (a bigger surface; this is just the inline picker line).
- Slash-command discovery — that's #2528.

## Workaround today

Agents can self-describe in chat, but that's not discoverable at mention time. A cheat-sheet has to be re-asked for.

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Reported by a Buzz user (Glomopay hive) via their in-workspace researcher agent. Screenshot is the user's own composer.

Start at the desktop @-mention autocomplete selector and trace how agent display names and avatars are rendered. Check how public kind:0 profiles provide the about field, then review agent creation and the kind:0 republish allowlist described in #2534. Done means rows show a truncated about line when present and retain name-only behavior when it is empty.

Written by the indexing model from the issue text.