Create database registry (PostgreSQL, Redis, MongoDB)

[varunreddy]

Summary

Add a database-focused registry with expert cards for PostgreSQL, Redis, and MongoDB.

Acceptance Criteria

• Create instruction files: postgresql.md, redis.md, mongodb.md in examples/registry/instructions/
• Each file follows the 8-section template (see docker.md for reference)
• Create database.registry.yaml in examples/registry/
• Add all 3 tools to tools.json and tools.yaml with full metadata
• Test retrieval with relevant queries (e.g., "optimize PostgreSQL query performance")
• All existing tests still pass