refactor: agent editor pane notes use Alert

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/common/paneNotice/PaneNotice.svelte

@@ -1,26 +0,0 @@
-<!--
-@component
-A one-line note across the top of a pane: why the pane behaves as it does, with at most one
-action that changes it. Lighter than an `Alert`, which is a box for a message of its own rather
-than a remark about the content under it.
--->
-<script lang="ts">
-	import { Info } from 'lucide-svelte'
-	import Button from '../button/Button.svelte'
-
-	interface Props {
-		icon?: typeof Info
-		action?: { label: string; onClick: () => void }
-		children: import('svelte').Snippet
-	}
-
-	let { icon: Icon = Info, action = undefined, children }: Props = $props()
-</script>
-
-<div class="shrink-0 flex items-center gap-2 px-4 py-1.5 border-b text-2xs text-secondary">
-	<Icon size={16} class="shrink-0" />
-	<span class="flex-1">{@render children()}</span>
-	{#if action}
-		<Button unifiedSize="2xs" variant="subtle" onClick={action.onClick}>{action.label}</Button>
-	{/if}
-</div>