feat: add Cursor marketplace and plugin support

[mmizutani]

Summary

Add .cursor-plugin/ manifests alongside the existing .claude-plugin/ ones so the Codex plugin is discoverable from both the Claude Code and Cursor marketplaces.

What's included

• .cursor-plugin/marketplace.json — root marketplace manifest for Cursor (uses pluginRoot convention, plugin-level metadata with category/tags/keywords)
• plugins/codex/.cursor-plugin/plugin.json — plugin manifest validated against cursor.com/docs/reference/plugins (only spec-valid fields; category/tags live in marketplace entry, not here)
• plugins/codex/hooks/cursor-hooks.json — Cursor-format hooks (version: 1, camelCase event names, timeout values, ${CURSOR_PLUGIN_ROOT} interpolation)
• plugins/codex/scripts/cursor-session-lifecycle-hook.mjs — Cursor sessionStart adapter that outputs { "env": { ... } } JSON (Cursor's mechanism) instead of writing to CLAUDE_ENV_FILE (Claude Code's mechanism)
• README.md — updated title, description, and install section with Cursor instructions

Design decisions

Concern	Approach
Hooks format differs (PascalCase/nested vs camelCase/flat)	Separate cursor-hooks.json referenced via explicit "hooks" field in plugin.json; existing hooks.json (Claude Code format) untouched
CURSOR_PLUGIN_ROOT vs CLAUDE_PLUGIN_ROOT	Hook commands bridge with CLAUDE_PLUGIN_ROOT="${CURSOR_PLUGIN_ROOT}" so existing scripts resolve paths correctly
CLAUDE_ENV_FILE not available in Cursor	Dedicated cursor-session-lifecycle-hook.mjs outputs { "env": { ... } } on stdout per Cursor hook output spec
CLAUDE_PLUGIN_DATA not available in Cursor	lib/state.mjs already falls back to $TMPDIR/codex-companion — no change needed
Shared components (skills, commands, agents)	Markdown-based — portable as-is; both manifests point to the same directories

Limitations

• The sessionEnd and stop hooks reuse the original scripts. They work but lack CLAUDE_ENV_FILE and CLAUDE_PLUGIN_DATA, so session-scoped env var propagation relies on Cursor's sessionStart → env output mechanism and the tmpdir state fallback.
• Command markdown files contain Claude Code-specific frontmatter (allowed-tools, disable-model-invocation). Cursor tolerates unknown frontmatter fields, but these features won't activate in Cursor.
• Cursor CLI does not support plugins yet (desktop only).

Cursor Marketplace/Plugins References

• https://cursor.com/docs/plugins

• https://cursor.com/docs/reference/plugins

• Examples:
  • https://cursor.com/marketplace/cursor/create-plugin
  • https://github.com/cursor/plugins

Test plan

• Install plugin via Cursor marketplace (/add-plugin openai/codex-plugin-cc) and verify it appears in installed plugins
• Verify /codex:setup, /codex:review, and other slash commands are discoverable in Cursor
• Verify sessionStart hook fires and propagates CODEX_COMPANION_SESSION_ID to subsequent hooks
• Verify existing Claude Code plugin functionality is unaffected (no changes to .claude-plugin/ or hooks/hooks.json)

Note

We might be able to interpret the *cc portion of the repo name as either "Claude Code" or "Claude Code and Cursor".

[mmizutani]

It seems Cursor Plugins skip support for Commands in favor of Skills. Fixing ...