feat: initial Spaghetti UE5 Agent Plugin (Agent Plugins 1.0.0)

Agent Plugins 1.0.0 distribution package for Spaghetti Unreal AI Workbench.

Structure:
- plugin.json: manifest (spaghetti-ue5 v0.1.0, Tinirau Ltd, MIT)
- mcp.json: streamable-http → https://mcp.spaghetti.gg (OAuth, no secrets)
- skills/ue5-unreal/SKILL.md: AI-facing tool routing for Crash Investigator,
  Asset Janitor, Smart Librarian, Tool-Maker + safety defaults
- README.md: install (npx plugins add), auth (OAuth browser flow), .spaghetti
  context note
- LICENSE: MIT

OAuth flow: client connects → 401 + WWW-Authenticate → RFC 9728 metadata →
GoTrue SSO → token. All server-side; no client-side secrets.

.spaghetti context file: {team_id, project_id} at the UE project root.
The mcp_server field is not included — mcp.json owns the URL.

plugin.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://agent-plugins.org/schemas/1.0.0/plugin.schema.json",
+  "name": "spaghetti-ue5",
+  "version": "0.1.0",
+  "description": "Unreal Engine 5 agent tools — crash analysis, asset auditing, knowledge base, code generation. Powered by Spaghetti Unreal AI Workbench.",
+  "author": {
+    "name": "Tinirau Ltd",
+    "url": "https://spaghetti.gg"
+  },
+  "homepage": "https://spaghetti.gg",
+  "repository": "https://git.tinirau.com/Tinirau/ue5-agent-plugin",
+  "license": "MIT",
+  "keywords": ["unreal", "ue5", "spaghetti", "mcp", "game-development", "ai"]
+}