fix: vendor per-kind sites in the hub; drop stale blog docs

- collect.py: when a kind points at the shared site root, copy only the
  per-kind subdir (site/lib -> <repo>/lib) instead of the whole site, so
  stray single-site build output never leaks into the hub
- Purge stale collected artifacts (404.html, assets/, search/, sitemaps,
  objects.inv, duplicate mcp/) from every repo dir, matching the cleaned
  source site/ roots
- Refresh collected output from source repos, incl. the khata-app app
- Remove the vendored Blog API docs (config.yml entry + blog/) and its card

dagpipe/mcp/nav.json

@@ -1,26 +0,0 @@
-[
-  {
-    "module": "dagpipe",
-    "resource": "doc://modules/dagpipe"
-  },
-  {
-    "module": "dagpipe.engine",
-    "resource": "doc://modules/dagpipe.engine"
-  },
-  {
-    "module": "dagpipe.graph",
-    "resource": "doc://modules/dagpipe.graph"
-  },
-  {
-    "module": "dagpipe.node",
-    "resource": "doc://modules/dagpipe.node"
-  },
-  {
-    "module": "dagpipe.state",
-    "resource": "doc://modules/dagpipe.state"
-  },
-  {
-    "module": "dagpipe.yaml_loader",
-    "resource": "doc://modules/dagpipe.yaml_loader"
-  }
-]