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

.codex-plugin/README.md

@@ -0,0 +1,47 @@
+# .codex-plugin — Codex Native Plugin for ECC
+
+This directory contains the **Codex plugin manifest** for Everything Claude Code.
+
+## Structure
+
+```
+.codex-plugin/
+└── plugin.json   — Codex plugin manifest (name, version, skills ref, MCP ref)
+.mcp.json         — MCP server configurations at plugin root (NOT inside .codex-plugin/)
+```
+
+## What This Provides
+
+- **125 skills** from `./skills/` — reusable Codex workflows for TDD, security,
+  code review, architecture, and more
+- **6 MCP servers** — GitHub, Context7, Exa, Memory, Playwright, Sequential Thinking
+
+## Installation
+
+Codex plugin support is currently in preview. Once generally available:
+
+```bash
+# Install from Codex CLI
+codex plugin install affaan-m/everything-claude-code
+
+# Or reference locally during development
+codex plugin install ./
+```
+
+## MCP Servers Included
+
+| Server | Purpose |
+|---|---|
+| `github` | GitHub API access |
+| `context7` | Live documentation lookup |
+| `exa` | Neural web search |
+| `memory` | Persistent memory across sessions |
+| `playwright` | Browser automation & E2E testing |
+| `sequential-thinking` | Step-by-step reasoning |
+
+## Notes
+
+- The `skills/` directory at the repo root is shared between Claude Code (`.claude-plugin/`)
+  and Codex (`.codex-plugin/`) — same source of truth, no duplication
+- MCP server credentials are inherited from the launching environment (env vars)
+- This manifest does **not** override `~/.codex/config.toml` settings