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/pages/KnowledgeBase.svelte

@@ -6,7 +6,7 @@
   import EntityGraph, { type GraphInfo } from '$lib/components/EntityGraph.svelte'
   import EntityDetailContent from '$lib/components/EntityDetailContent.svelte'
   import MultiSelectFilter from '$lib/components/MultiSelectFilter.svelte'
-  import { categories, domainsForCategory, type Category } from '$lib/categories'
+  import { categories, filtersForCategory, type Category } from '$lib/categories'
   import { Button } from '$lib/components/ui/button'
   import { Input } from '$lib/components/ui/input'
   import * as Select from '$lib/components/ui/select'
@@ -47,8 +47,8 @@
 
   async function loadTable() {
     tableLoading = true
-    const domains = domainsForCategory(category)
-    const results = await Promise.all(domains.map((domain) => fetchEntities({ domain })))
+    const filterSets = filtersForCategory(category)
+    const results = await Promise.all(filterSets.map((f) => fetchEntities(f)))
     tableEntities = results.flat()
     tableLoading = false
   }