Add Common Issues table to databricks-unity-catalog skill

[CheeYuTan]

Summary

• Adds a 10-entry Common Issues troubleshooting table to the databricks-unity-catalog skill
• Covers system table permissions, query performance, volume paths, GRANT failures, lineage gaps, audit log delays, and billing usage interpretation

Test proof

Tested UC patterns against live workspace e2-demo-field-eng (warehouse bolun_sql_warehouse):

Test	Result
System table: system.access.audit (with date filter)	PASS — top action: tokenLogin (8.9M events)
System table: system.billing.usage (with date filter)	PASS — top SKU: ENTERPRISE_SERVERLESS_REAL_TIME_INFERENCE (527K DBUs)
System table: system.access.table_lineage	PASS — lineage records returned
GRANT syntax: USE CATALOG vs SELECT	PASS — verified distinction
Volume path format verification	PASS — /Volumes/main/default/spark-rtm-demo
Information schema: list schemas	PASS — schemas in main catalog listed
Create/drop test schema	PASS

TEST: System table: access.audit (with date filter)
  PASS
  → ['tokenLogin', '8945024']
  → ['oidcTokenAuthorization', '3160509']

TEST: System table: billing.usage (with date filter)
  PASS
  → ['ENTERPRISE_SERVERLESS_REAL_TIME_INFERENCE_US_WEST_OREGON', '527101.66']

TEST: Volume path format verification
  PASS
  → ['main', 'default', 'spark-rtm-demo', '/Volumes/main/default/spark-rtm-demo']

All 7/7 UC tests passed.

[calreynolds]

Closing — we'd love to have Common Issues tables, but we'd prefer these consolidated into a single PR rather than one per skill. Feel free to resubmit as a single combined PR if you're up for it!