fix: sync published content with the current platform (context contract, tool roster, version)

- README project context: .spaghetti file (rejected design) →
  Config/DefaultGame.ini [/Script/Spaghetti.SpaghettiProjectSettings]
  via the editor connector's Project Settings; "setup CLI is coming"
  dropped (CLI is reserve) in favor of roadmap-neutral wording
- SKILL.md: prerequisites now describe the ini section and the
  personal-account fallback (the old "Agentic Directive creates
  .spaghetti" mechanism never shipped); tool roster renamed to the
  live server's (analyze_logs, tidy_assets, five free research tools
  incl. project-scoped temporal ones); context/billing section
  updated with the 402=credits vs 403=sign-in/onboarding taxonomy
- plugin.json: version 0.1.1 → 0.3.1, tracking the editor connector

mcp.json unchanged — URL-only config is already correct for the
first-party AS (CIMD/DCR discovery, no credentials in the manifest).

plugin.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://agent-plugins.org/schemas/1.0.0/plugin.schema.json",
   "name": "unreal-workbench",
-  "version": "0.1.1",
+  "version": "0.3.1",
   "description": "Unreal Engine 5 agent tools — log analysis, asset auditing, engine docs research, script generation. Powered by Spaghetti Unreal AI Workbench.",
   "author": {
     "name": "Tinirau Ltd",