feat(desktop): show each agent's role/description in the @-mention selector #2699

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 feat(desktop): slash-command autocomplete in composer from ACP available_commands_update (follow-up to #919) #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: kind:0 profile republish drops every field outside a 5-key allowlist — silently deletes bot: true, nip05 and website #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 feat(desktop): slash-command autocomplete in composer from ACP available_commands_update (follow-up to #919) #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.

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