feat: add native Cursor plugin adapter

[harshav167]

Summary

• Add a Cursor plugin manifest that exposes Ponytail skills, existing Cursor rules, and Cursor-native hooks.
• Add Cursor hook adapters for session activation, mode tracking, dependency-install approval, compaction reminders, and session cleanup.
• Preserve existing Claude/Codex/Copilot hook behavior while adding Cursor output/state handling.

Test plan

• node tests/hooks.test.js
• node tests/opencode-plugin.test.js
• node scripts/check-rule-copies.js
• node tests/hooks-windows.test.js
• node tests/gemini-extension.test.js
• node tests/behavior.test.js
• node tests/correctness.test.js
• Node smoke checks for Cursor hook JSON outputs