fix(knowledge): populate hit id in search results

Search hits and entity-knowledge hits never selected an id column, so
every KnowledgeHit.Id defaulted to the zero UUID. The frontend's keyed
{#each results as hit (hit.id)} then had all-duplicate keys, which
silently broke Svelte 5's if-block branch swap for the results panel —
search would set searched=true (Clear button appeared) but the view
never switched away from "Recently learned". Select e.id in both
queries and key the each block on hit.slug (guaranteed unique) instead.

web/src/pages/Knowledge.svelte

@@ -115,7 +115,7 @@
     <p class="text-sm text-muted-foreground">{results.length} result{results.length === 1 ? '' : 's'} for "{query}"</p>
     <ScrollArea class="flex-1">
       <div class="flex flex-col gap-3 pr-4">
-        {#each results as hit (hit.id)}
+        {#each results as hit (hit.slug)}
           <Card.Root class="transition-colors hover:bg-muted/50">
             <Card.Header>
               <div class="flex items-center gap-2">