feat(codex): add Codex native plugin manifest and fix Claude plugin.json

- Add .codex-plugin/plugin.json — Codex-native plugin manifest with
  skills reference and MCP server config pointer
- Add .codex-plugin/.mcp.json — standalone MCP server config bundle
  (github, context7, exa, memory, playwright, sequential-thinking)
- Add .codex-plugin/README.md — installation guide and server reference
- Fix .claude-plugin/plugin.json — add missing agents[] (28 explicit
  file paths per validator rules), skills[], and commands[] arrays;
  remove hooks field (auto-loaded by Claude Code v2.1+ convention)
- Add tests/plugin-manifest.test.js — 16 CI tests enforcing
  PLUGIN_SCHEMA_NOTES.md rules (no hooks, arrays throughout, explicit
  agent paths, version required, .mcp.json structural checks)
- Update package.json: add .codex-plugin/ to files[], add plugin
  manifest test to npm test chain

Refs: .claude-plugin/PLUGIN_SCHEMA_NOTES.md

.mcp.json

@@ -0,0 +1,27 @@
+{
+  "mcpServers": {
+    "github": {
+      "command": "npx",
+      "args": ["-y", "@modelcontextprotocol/server-github"]
+    },
+    "context7": {
+      "command": "npx",
+      "args": ["-y", "@upstash/context7-mcp@2.1.4"]
+    },
+    "exa": {
+      "url": "https://mcp.exa.ai/mcp"
+    },
+    "memory": {
+      "command": "npx",
+      "args": ["-y", "@modelcontextprotocol/server-memory"]
+    },
+    "playwright": {
+      "command": "npx",
+      "args": ["-y", "@playwright/mcp@0.0.68", "--extension"]
+    },
+    "sequential-thinking": {
+      "command": "npx",
+      "args": ["-y", "@modelcontextprotocol/server-sequential-thinking"]
+    }
+  }
+}