docs: phase 0 analysis - architecture, plugin api requirements, sync and release design

.opencode/state/upstream.json

@@ -0,0 +1,8 @@
+{
+  "repo": "https://github.com/mattpocock/skills",
+  "commit": "49dd158d1076134a641b33efb035946536778336",
+  "commit_date": "2026-10-09T11:56:49+01:00",
+  "upstream_version": "1.3.1",
+  "synced_at": null,
+  "note": "HEAD observed during phase-0 analysis; upstream/ snapshot not vendored yet"
+}