Expand Structured Streaming SKILL.md from table of contents to actionable hub

[CheeYuTan]

Summary

The Structured Streaming SKILL.md was only 66 lines — just a table of contents despite having 8 good reference files. Expanded to 247 lines with actionable content:

• Quick starts: Kafka-to-Delta, foreachBatch MERGE, availableNow scheduled streaming
• Trigger selection guide: realTime (DBR 16.4+), processingTime, availableNow, once
• Watermark essentials with duration selection table
• Stream join patterns: stream-stream with time bounds, stream-static with broadcast hints
• Checkpoint best practices summary
• Production checklist and common issues troubleshooting table

No new files — just an expanded SKILL.md that makes the existing reference files more discoverable and useful.

Test plan

• All trigger syntax verified against PySpark docs
• No install_skills.sh changes needed (no new files)
• Cross-referenced with existing reference files for consistency

[CheeYuTan]

Test Results

Test	Status	Details
CI Validation (validate_skills.py)	PASS	All 26 skills validated
SKILL.md Frontmatter	PASS	name: 40 chars, lowercase+hyphens. description: 189 chars, no XML
Reference File Cross-links	PASS	All 9 existing reference files linked in SKILL.md
Trigger Syntax	PASS	processingTime="30 seconds", availableNow=True, realTime="5 minutes" (DBR 16.4+) — all correct
Watermark Syntax	PASS	.withWatermark("event_time", "10 minutes") — correct
foreachBatch MERGE	PASS	DeltaTable.forName() → .merge() → .whenMatchedUpdateAll() → .execute() — correct
Stream-Stream Join	PASS	Uses expr() with SQL INTERVAL for time bounds — correct
install_skills.sh	PASS	No changes needed (no new files added)

What Changed

SKILL.md expanded from 66 → 247 lines. Was just a table of contents; now includes actionable quick starts, trigger selection guide, watermark essentials, stream join patterns, checkpoint best practices, and common issues table. All 9 existing reference files are now more discoverable.

Advisory Note

Pre-existing reference files (trigger-and-cost-optimization.md, kafka-streaming.md) still use the older realTime=True syntax. This PR's SKILL.md correctly uses realTime="5 minutes". A follow-up PR could update those reference files for consistency.