feat(cursor): add Hindsight memory plugin for Cursor

[DK09876]

Summary

• Adds a complete Cursor integration using the plugin architecture (hooks, skills, rules)
• Auto-recall via beforeSubmitPrompt hook injects relevant memories as additionalContext
• Auto-retain via stop hook captures conversation transcripts to Hindsight
• Includes on-demand hindsight-recall skill and hindsight-memory rule
• Full lib/ directory (config, bank, state, daemon, client, content, llm) — modeled after claude-code
• 41 tests covering config loading, bank derivation, content processing, hook behavior, and manifest validation
• CI job (test-cursor-integration) with change detection
• Added to VALID_INTEGRATIONS in release script

Test plan

• All 41 tests pass locally (python -m pytest tests/ -v)
• CI passes on this PR
• Manual smoke test: install plugin in Cursor, verify recall/retain work end-to-end

🤖 Generated with Claude Code