chore: remove unnecessary files and clean up project

chore: enable CLAUDE_PROCESS_TIMEOUT and SESSION_CLEANUP_DAYS defaults

docs: update .env.example defaults and add .env

docs: update README demo image to local public/demo.png

docs: update CLAUDE.md, CONTRIBUTING.md, and README.zh-TW.md

feat(discord,slack): add optional CHANNEL_IDS filtering

feat(discord): support multiple allowed user IDs (comma-separated)

fix(email): include [project-name] in all email subjects for reply threading

docs: add complete Slack DM and slash command setup instructions

docs: update README and add Slack /project slash command

refactor: replace Cancel button with close confirmation dialog

fix: replace Send button with Cancel when task is running

fix: improve mobile layout for Web UI

feat: add Claude API mode as alternative to CLI mode

fix: add /cancel to help messages for Telegram, LINE, and Email