fix: an item the picker files under no kind keeps its name in the band

The merge brought main's agents detail page, which reaches DetailPageHeader with
no `errorHandlerKind` — and the band's item required one. `WorkspaceItemKindIcon`
falls through to the script glyph for anything it does not know, so defaulting
would have labelled an agent a script; the kind is optional now and the trail
names the item without claiming it is one of the three.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/NavBreadcrumb.svelte

@@ -148,7 +148,9 @@ same weight, size and icon size, so the line reads as one control rather than fo
 {/snippet}
 
 {#snippet itemKindIcon()}
-	{#if item}<WorkspaceItemKindIcon kind={item.kind} size={ICON} />{/if}
+	<!-- No kind, no glyph: WorkspaceItemKindIcon falls through to the script icon, which would
+	     label an agent a script. -->
+	{#if item?.kind}<WorkspaceItemKindIcon kind={item.kind} size={ICON} />{/if}
 {/snippet}
 
 {#snippet sectionKindIcon()}
@@ -304,7 +306,7 @@ same weight, size and icon size, so the line reads as one control rather than fo
 					</span>
 				{/if}
 			</div>
-		{:else}
+		{:else if item.kind}
 			<span class="{SEGMENT} hover:bg-transparent" aria-current="page">
 				{@render itemKindIcon()}
 				<span class="truncate">{KIND_LABEL_LOWER[item.kind]}</span>

frontend/src/lib/components/details/DetailPageHeader.svelte

@@ -35,9 +35,7 @@
 
 	// An agent's page has no triggers, and so no context for them.
 	const triggerContext = getContext<TriggerContext | undefined>('TriggerContext')
-	const { triggersCount, triggersState } = $state(
-		triggerContext ?? ({} as Partial<TriggerContext>)
-	)
+	const { triggersCount, triggersState } = $state(triggerContext ?? ({} as Partial<TriggerContext>))
 	// The buttons below render in the page header, away from this page's tree.
 	const headerContexts = new Map<any, any>([['TriggerContext', triggerContext]])
 

frontend/src/lib/components/pageHeaderRegistry.svelte.ts

@@ -3,7 +3,9 @@ import type { WorkspaceItemKind } from './workspacePicker'
 
 /** An item the page is working on: the breadcrumb walks its path and ends with its summary. */
 export type PageHeaderItem = {
-	kind: WorkspaceItemKind
+	/** Absent for an item the picker does not file under a kind — an agent's detail page. The
+	 *  breadcrumb then names the item without claiming it is one of the three. */
+	kind?: WorkspaceItemKind
 	/** Shown whole, and copied on click. */
 	path: string | undefined
 	summary?: string