release: 2.0.0 — the agent harness operating system

Graduate 2.0.0-rc.1 to stable. Bump version across package, plugin,
marketplace, OpenCode, agent metadata, VERSION, and all localized docs.
Add 2.0.0 release notes + README sections (en/zh/pt-BR/tr), CHANGELOG
entry, and the ECC community Discord bot (dependency-free gateway client
+ guild command registrar). Update copilot-support and release-surface
tests for the sponsored-review migration and the 2.0.0 surface.

.agents/plugins/marketplace.json

@@ -6,7 +6,7 @@
   "plugins": [
     {
       "name": "ecc",
-      "version": "2.0.0-rc.1",
+      "version": "2.0.0",
       "source": {
         "source": "local",
         "path": "./"