feat(desktop): clean up agents page UI

[wesbillman]

Summary

Cleans up the agents page UI based on feedback — smaller buttons, polling instead of manual refresh, compact persona cards, and improved dark-mode readability.

Changes

1. Compact Create buttons

• Both Personas and Managed Agents sections: full-size Create buttons replaced with icon-only + ghost buttons
• Tooltip on hover shows "Create persona" / "Create agent"
• aria-label added for accessibility

2. Polling replaces Refresh buttons

• Managed agents already polled (2s running / 10s idle) — Refresh button removed
• Personas — added refetchInterval: 30_000 (30s), Refresh button removed
• Relay agents — added refetchInterval: 15_000 (15s)
• handleRefresh function and all onRefresh props removed from AgentsView

3. Compact persona cards

• Grid density increased: md:grid-cols-3 xl:grid-cols-5 (was 2/4)
• Avatars: h-8 w-8 (was 10x10), padding: p-2 (was p-3)
• Inline prompt preview removed — replaced with ℹ️ icon tooltip showing system prompt on hover
• Empty prompts handled gracefully (no tooltip rendered)

4. Dark-mode readability in Create Agent dialog

• Toggle option descriptions: text-muted-foreground → text-foreground/70
• Scope options now include descriptions (e.g. "Read messages in joined channels") rendered below each label with text-foreground/60

Files changed (6)

• hooks.ts — added refetchInterval to usePersonasQuery and useRelayAgentsQuery
• AgentsView.tsx — removed handleRefresh, removed onRefresh props
• ManagedAgentsSection.tsx — icon-only create button, removed refresh, cleaned up dead onModelChanged prop
• PersonasSection.tsx — icon-only create, compact cards, ℹ️ tooltip for prompt preview
• CreateAgentDialogSections.tsx — dark-mode text contrast fix, scope descriptions
• scopeOptions.ts — added description field to all scope options

Testing

• tsc --noEmit ✅
• biome check ✅
• Pre-push CI: fmt, clippy, desktop-build, tauri-check, rust-tests ✅