Add Claude Skill support, plugin manifests, and plugin folder updates

plugins/.codex-plugin/marketplace.json

@@ -0,0 +1,9 @@
+{
+  "plugins": [
+    {
+      "name": "semantica-codex",
+      "description": "Semantica plugin for Codex: knowledge graph commands and analytics.",
+      "path": "."
+    }
+  ]
+}

plugins/.codex-plugin/plugin.json

@@ -0,0 +1,11 @@
+{
+  "name": "semantica-codex",
+  "description": "Semantica plugin for Codex: knowledge graph commands, export capabilities, and reasoning workflows.",
+  "version": "0.1.0",
+  "author": {"name": "Semantica Contributors"},
+  "homepage": "https://github.com/Hawksight-AI/semantica",
+  "repository": "https://github.com/Hawksight-AI/semantica",
+  "platforms": ["codex"],
+  "keywords": ["semantica", "knowledge graph", "codex", "visualization", "reasoning", "extraction", "mcp"],
+  "license": "MIT"
+}