Add databricks-dqx skill for Data Quality Extensions

[aniljoshiaset]

Summary

• Adds comprehensive skill for Databricks Labs DQX (Data Quality Extensions) framework
• Covers quality rule definition, profiling, auto-generation, Lakeflow/DLT integration, streaming, and check functions
• SKILL.md (256 lines) with 7 progressive-disclosure reference files following skill authoring best practices

Skill Structure

databricks-skills/databricks-dqx/
├── SKILL.md                          # Main skill (256 lines)
├── 1-installation-setup.md           # Install methods (pip, CLI, DABs, extras)
├── 2-defining-quality-rules.md       # DQRule classes, YAML metadata, storage
├── 3-applying-checks.md              # Apply checks, split valid/invalid, save
├── 4-profiler-auto-generation.md     # Profile datasets, auto-generate rules
├── 5-lakeflow-integration.md         # DLT/Lakeflow pipeline integration
├── 6-streaming-metrics.md            # Streaming support, quality monitoring
└── 7-check-functions-reference.md    # Complete built-in check function catalog

Test plan

• SKILL.md has valid YAML frontmatter with name and description
• Description is third-person, includes what the skill does and when to use it
• Under 500 lines (256)
• Progressive disclosure with reference files one level deep
• No time-sensitive information
• All file paths use forward slashes
• Updated databricks-skills/README.md skills table
• Updated main README.md skill count (19 → 20)

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