Update docs for OpenAI provider support

Release 1.6.0

Add OpenAI provider and unify AI API layer

Store diaries in year/month folders

Remove duplicate title and requirements from release notes

Release 1.5.0

Add Codex CLI/App support as activity source

Fix documentation errors and add missing content

Remove Claude CLI from AI provider documentation

Add Cursor support mention to README

Update README style and add Japanese version

Release 1.4.1

Release 1.4.0

Release 1.3.1

Remove Claude Code CLI provider option (#6)