feat: Add configuration validation command

[db-tycoon-stephen]

Summary

Add uv run validate-config command to validate SnowDDL YAML configurations before deployment.

Motivation

From roadmap Priority 2 (Short-term Improvements):

• Create shared configuration schema
• Implement config validation across configs
• Catch errors before deployment

Proposed Features

1. Schema validation: Verify YAML structure matches expected format
2. Reference validation: Check that referenced roles, warehouses, databases exist
3. Permission validation: Verify grant chains are valid
4. Duplicate detection: Find duplicate user/role definitions
5. Warning mode: Non-blocking issues vs blocking errors

Implementation

uv run validate-config              # Validate all configs
uv run validate-config --strict     # Fail on warnings too
uv run validate-config snowddl/user.yaml  # Validate specific file

Acceptance Criteria

• Command registered in pyproject.toml
• Validates user.yaml, tech_role.yaml, business_role.yaml
• Reports errors with file:line references
• Exit code 0 on success, 1 on errors
• Unit tests for validation logic

[db-tycoon-stephen]

Completed in PR #26 (merged to v0.3)