feat: Adds React 18 and 19 migration plugin (#1339)

- Adds React 18 and 19 migration orchestration plugins
- Introduces comprehensive upgrade toolkits for migrating legacy React 16/17 and 18 codebases to React 18.3.1 and 19, respectively. Each plugin bundles specialized agents and skills for exhaustive audit, dependency management, class/component API migration, test suite transformation, and batching regression fixes.
- The React 18 toolkit targets class-component-heavy apps, ensures safe lifecycle and context transitions, resolves dependency blockers, and fully automates test migrations including Enzyme removal. The React 19 toolkit addresses breaking changes such as removal of legacy APIs, defaultProps on function components, and forwardRef, while enforcing a gated, memory-resumable migration pipeline.
- Both plugins update documentation, plugin registries, and skill references to support reliable, repeatable enterprise-scale React migrations.

plugins/react19-upgrade/.github/plugin/plugin.json

@@ -0,0 +1,30 @@
+{
+  "name": "react19-upgrade",
+  "description": "Enterprise React 19 migration toolkit with specialized agents and skills for upgrading React 18 codebases to React 19. Includes auditor, dependency surgeon, source code migrator, and test guardian. Handles removal of deprecated APIs including ReactDOM.render, forwardRef, defaultProps, legacy context, string refs, and more.",
+  "version": "1.0.0",
+  "keywords": [
+    "react19",
+    "react",
+    "migration",
+    "upgrade",
+    "hooks",
+    "modern-react"
+  ],
+  "author": {
+    "name": "Awesome Copilot Community"
+  },
+  "repository": "https://github.com/github/awesome-copilot",
+  "license": "MIT",
+  "agents": [
+    "./agents/react19-auditor.md",
+    "./agents/react19-commander.md",
+    "./agents/react19-dep-surgeon.md",
+    "./agents/react19-migrator.md",
+    "./agents/react19-test-guardian.md"
+  ],
+  "skills": [
+    "./skills/react19-concurrent-patterns/",
+    "./skills/react19-source-patterns/",
+    "./skills/react19-test-patterns/"
+  ]
+}