feat: mirror tag-vocabulary summary and note-type validation

Ports two changes from the knowledge-base vault this skeleton is extracted
from, keeping the shared scripts byte-identical across the pair:

- `graph_summary` reports `by_tag` and `tag_count`, sized by `topTags`, and
  drops the `by_type`/`by_status` breakdowns. An MCP-only agent could not
  otherwise discover the tag spellings that `graph_search_nodes` matches
  verbatim.
- `generate-graph.py --validate` now enforces a controlled vocabulary for a
  note's `type`, which is where the drift signal `by_type` used to give now
  lives. `AGENT_QUERY_GUIDE.md`, both skills and the-knowing-one quote the
  enforced enum.

`tests/test_generate_graph.py` is new; `make test` is 50 tests. Regenerating
the graph also drops a stale `GLOBAL_AGENTS` node for a file this skeleton
does not carry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

index/graph.json

@@ -1,6 +1,6 @@
 {
   "version": 1,
-  "generated_at": "2026-09-11T07:34:35.807809+00:00",
+  "generated_at": "2026-09-14T09:33:34.687166+00:00",
   "nodes": [
     {
       "id": ".claude/agents/the-knowing-one",
@@ -50,18 +50,6 @@
       "outbound_links": [],
       "inbound_count": 0
     },
-    {
-      "id": "GLOBAL_AGENTS",
-      "title": "GLOBAL_AGENTS.md",
-      "path": "GLOBAL_AGENTS.md",
-      "repo": "global",
-      "type": "note",
-      "status": "active",
-      "tags": [],
-      "last_reviewed": "",
-      "outbound_links": [],
-      "inbound_count": 0
-    },
     {
       "id": "agents/README",
       "title": "Agents",