Add Snowflake AI prompt rules for SQL, Cortex AI, Snowpark Python, and dbt

[jamescha-earley]

Summary

• Adds 4 Snowflake prompt rule sets — the first data warehouse/data platform rules in the repository.
• snowflake-sql: SQL best practices, data pipelines (Dynamic Tables, Streams, Tasks, Snowpipe), semi-structured data, performance optimization, cost management.
• snowflake-cortex: Cortex AI Functions (AI_COMPLETE, AI_CLASSIFY, AI_EXTRACT, AI_EMBED, etc.) and Cortex Search for hybrid vector+keyword search and RAG.
• snowflake-python: Snowpark Python server-side DataFrames, scalar/vectorized UDFs, UDTFs, stored procedures, pandas on Snowflake.
• snowflake-dbt: dbt-snowflake adapter with dynamic table materialization, incremental models, Snowflake-specific configs.

All content is grounded in official Snowflake documentation.

Each directory follows the CONTRIBUTING.md format:

• aiprompt.json with techStack (camelCase), type: "rule", and .mdc file references
• rule-snowflake-{name}.mdc with YAML frontmatter (description, globs) and markdown body

Checklist

• Each prompt is in its own prompts/snowflake-{name}/ directory
• Each contains aiprompt.json + rule-snowflake-{name}.mdc
• Follows CONTRIBUTING.md format conventions