[+] add experiment "chainer" AI-agent for pg_timetable chain management

Introduces a new AI-agent specialized in creating, designing, and
troubleshooting pg_timetable scheduled job workflows in PostgreSQL.

The agent provides comprehensive documentation on:

- Database schema and core architecture
- Command types (SQL, PROGRAM, BUILTIN) with examples
- Helper functions and scheduling formats
- Common patterns and best practices
- Testing and development workflows
- Error handling and troubleshooting

This enables users to leverage AI assistance for implementing complex
chains, multi-task workflows, and data-driven job scheduling directly
in PostgreSQL.

Author: pashagolub