feat: learning view — make the growing knowledge base visible

First slice of the observability/learning UI (the "see the system come alive
and learn" ask). The Knowledge page was search-only — blank until you typed —
so the knowledge Nomos now writes via upsert_knowledge was invisible unless
you knew to search for it. Now the page LEADS with what the system knows and
is learning:

- internal/httpapi/knowledge.go: GET /api/v1/knowledge/recent — recency-ordered
  knowledge + a stats header (total, agent-authored, learned-this-week,
  by-kind). Custom route (not OpenAPI-generated), same auth as the rest.
- web Knowledge page rewrite: stat cards up top (Total / Written by Nomos /
  Learned this week / runbooks-investigations), then a "Recently learned" feed
  with agent-authored notes highlighted and badged "learned by Nomos", tags,
  and relative timestamps. A toggle filters to Nomos-only. Search still works,
  now as a mode you enter/clear rather than the whole page.

This turns "the system is getting smarter" from a claim into something you
watch fill up: every gotcha the agent records shows here within seconds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/api.ts

@@ -378,6 +378,36 @@ export interface KnowledgeHit {
   slug: string
   type: 'document' | 'runbook' | 'investigation'
   title: string
+  snippet?: string
+  linked_entities?: string[]
+}
+
+export interface KnowledgeItem {
+  slug: string
+  title: string
+  kind: 'document' | 'runbook' | 'investigation'
+  source: string
+  tags: string[]
+  updated_at: string
+  agent_authored: boolean
+}
+
+export interface RecentKnowledge {
+  stats: {
+    total: number
+    by_kind: Record<string, number>
+    agent_authored: number
+    last_7d: number
+  }
+  items: KnowledgeItem[]
+}
+
+export async function fetchRecentKnowledge(source?: string): Promise<RecentKnowledge> {
+  const params = new URLSearchParams()
+  if (source) params.set('source', source)
+  const res = await fetch(`${API}/knowledge/recent?${params}`)
+  if (!res.ok) return { stats: { total: 0, by_kind: {}, agent_authored: 0, last_7d: 0 }, items: [] }
+  return res.json()
 }
 
 export async function fetchEntityKnowledge(entityId: string): Promise<KnowledgeHit[]> {