fix: show agent descriptions in @ autocomplete menu - #35321

Problem

When typing @ in the TUI prompt, the autocomplete menu shows agent names but not their descriptions. This makes it hard to know what each agent does without memorizing them or checking the config.

Root Cause

The agents createMemo in packages/tui/src/component/prompt/autocomplete.tsx creates AutocompleteOption objects without the description field, even though:

1. AutocompleteOption type supports description?: string
2. The rendering code already renders descriptions via <Show when={option().description}>
3. All other option sources (commands, referenceAliases, mcpResources) already pass description

Fix

One-line change: add description: agent.description to the agent AutocompleteOption mapping.

Before

(agent): AutocompleteOption => ({
    display: "@" + agent.name,
    onSelect: () => { ... },
})

After

(agent): AutocompleteOption => ({
    display: "@" + agent.name,
    description: agent.description,
    onSelect: () => { ... },
})

Verification

The rendering logic (lines at bottom of the file) already handles descriptions:

<Show when={option().description}>
    <text ...>
        {" " + option().description?.trimStart()}
    </text>
</Show>