fix(web): render full document content, keep graph connected under categories

Two fixes to the new category taxonomy:

- Knowledge Base couldn't show a document/investigation/runbook's own
  markdown body — knowledge_entities.content was never exposed by any
  endpoint (GetEntityKnowledge answers "what knowledge references this
  entity", not "what is this entity's content"). Add GET
  /api/v1/knowledge/content/{id} and render it with the existing
  marked+DOMPurify pipeline in a new Content section.

- The graph hid any edge whose other endpoint wasn't in the active
  category, so nodes with only cross-category neighbors rendered as
  disconnected dots. Queried the real relationship table: ~70% of infra
  edges cross Fleet/Network/Services/Storage lines (compute+network+
  software+storage+physical used to be one "infrastructure" layer).
  EntityGraph now keeps 1-hop neighbors visible but dimmed instead of
  hiding them, so the edges — and what they connect to — stay visible.

- categories.ts: `cognition` domain conflated true knowledge (document/
  investigation/runbook, 58 entities) with operational telemetry
  (execution/check/task/signal/approval/pattern/skill/classification/
  feedback, 300+ entities with their own Operations/Signals/Learning
  pages). Mapping the whole domain to Knowledge pulled in 245 execution
  entities fanning out from ~17 compute nodes via `targets` edges — the
  single biggest source of graph clutter. Knowledge now maps by type
  (document/investigation/runbook only); the rest of cognition is
  excluded from Knowledge Base browsing entirely.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/api.ts

@@ -601,6 +601,23 @@ export async function fetchEntityKnowledge(entityId: string): Promise<KnowledgeH
   return data.items ?? []
 }
 
+export interface KnowledgeContent {
+  title: string
+  content: string
+  source: string
+  tags: string[]
+  updated_at: string
+}
+
+// Full markdown body for a document/investigation/runbook entity — distinct
+// from fetchEntityKnowledge, which returns knowledge that references OTHER
+// entities, not this entity's own content.
+export async function fetchKnowledgeContent(id: string): Promise<KnowledgeContent | null> {
+  const res = await fetchWithAuth(`${API}/knowledge/content/${encodeURIComponent(id)}`)
+  if (!res.ok) return null
+  return res.json()
+}
+
 export async function fetchEntityEvents(entityId: string): Promise<import('./stores/events').OikosEvent[]> {
   const params = new URLSearchParams({ entity_id: entityId, limit: '50' })
   const res = await fetchWithAuth(`${API}/events?${params}`)