platforms: add the three platform guides the /platforms/ page renders

The /platforms/ page (408d6cd) reads platforms/*.yml and fails the build when
they are absent. Those three files were introduced on feat/phase-4-ingestion by
the ingestion commit 1e46ec0 and nothing else on the site needed them, so the
cherry-pick of 408d6cd left the tree unbuildable.

They are data, not ingestion code: each file is config plus public copy that the
site shows verbatim so a visitor can see exactly what the assistant reads. No
corpus/, no harvester, no Prefect flow and no Qdrant code is included.

Taken verbatim from 1e46ec0.

go test ./...: pass.

platforms/graphify.yml

@@ -0,0 +1,23 @@
+# Platform guide — Graphify knowledge graph.
+#
+# This file is both configuration and public copy.
+name: graphify
+enabled: true
+what: >-
+  A knowledge graph over the notes. It answers "what connects X to Y" rather
+  than "what mentions X": nodes are the things in the estate, edges are the
+  relations between them, and every one points back at the note it came from.
+fits: [ai]
+base: sam/obsidian-vault
+read: [graph_json]
+public: true
+limits:
+  max_documents: 4000
+  # graph.json is the only file this platform reads. These are the places it may
+  # be committed; the first match wins.
+  graph_paths:
+    - graphify-out/graph.json
+    - graph.json
+    - Graph/graph.json
+  max_nodes: 20000
+  max_edges: 40000