feat: add transcript header with directory name and timestamp range

- Update EntrySchema to include metadata fields (cwd, timestamp, sessionId, etc.)
- Capture meta entry and track timestamps during JSONL parsing
- Generate header with Claude Code Transcript title, directory name, and time range
- Format timestamps to UTC seconds precision (without milliseconds)
- Display header before transcript content for better context

🤖 Generated with [Claude Code](https://claude.ai/code)

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