Add Codex plugin marketplace support

[kpeez]

Summary

• add a native Codex marketplace manifest
• package the existing skills as an obsidian plugin
• preserve Claude Code marketplace support using the shared plugin directory
• add the official Obsidian logo and Codex plugin metadata
• document installation for Claude Code and Codex

Repository structure

This reorganizes the existing skills under a shared plugin root to work with the Codex plugin format:

plugins/obsidian/
├── .claude-plugin/plugin.json
├── .codex-plugin/plugin.json
├── assets/
└── skills/

Both Claude Code and Codex marketplaces now reference the same plugin payload. This avoids duplicating the skills while supporting each platform’s native manifest format. The skill contents themselves are unchanged; only their paths were moved.

Closes #65