Add Snowflake best practices rules

[jamescha-earley]

Summary

• Add snowflake entry to rules.json with database/data-warehouse/cloud/sql/analytics tags
• Add rules-mdc/snowflake.mdc with comprehensive Snowflake best practices

What the rules cover

• SQL fundamentals: naming conventions, CTEs, MERGE, stored procedures (colon prefix for variables)
• Semi-structured data: VARIANT/OBJECT/ARRAY, FLATTEN, null handling
• Performance: cluster keys, Search Optimization Service, warehouse sizing
• Data pipelines: Dynamic Tables, Streams+Tasks, Snowpipe (with decision table for choosing approach)
• Cortex AI Functions: all 13 AI_* functions, models, examples, cost awareness
• Cortex Search: CREATE SERVICE syntax (single/multi-index), Python query API, RAG pattern
• Snowpark Python: Session, DataFrame API, scalar vs vectorized UDFs, stored procedures
• dbt on Snowflake: dynamic_table materialization, Snowflake-specific configs
• Security: RBAC, masking policies, credential management
• Common pitfalls: anti-patterns across all areas

All content grounded in official Snowflake documentation.

.... Generated with Cortex Code