Prepare for public repo: gitignore prompts.json, add examples

- prompts.json now gitignored (contains account-specific config)
- Added prompts.example.json showing config structure
- Added .env.example with credential template

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -4,3 +4,4 @@ node_modules/
 history.json
 history-*.json
 bot.log
+prompts.json