Add battle-tested improvements to app-python, app-apx, and model-serving skills

[CheeYuTan]

Summary

Adds real-world fixes and troubleshooting guidance to 3 skills, based on hands-on experience deploying Databricks Apps in production.

databricks-app-python (6 files, +8.4KB)

Critical Rules (SKILL.md):

• /metrics endpoint requirement — gateway health-checks this endpoint
• CDN blocking — app runtime blocks outbound CDN requests (React, Recharts, Google Fonts, Babel)
• Don't delete-recreate apps — just redeploy; deleting disrupts OAuth integration
• Don't upload dev files (node_modules/, __pycache__/, frontend/src/)
• React hooks ordering — hooks before early returns to avoid Error #310

Resource Management (2-app-resources.md):

• Fix valueFrom YAML syntax (valueFrom.resource: instead of flat valueFrom:)
• Add CLI API PATCH workflow for attaching resources programmatically
• Add valueFrom vs value fallback troubleshooting

Framework Guidance (3-frameworks.md):

• Add DATABRICKS_APP_PORT details per framework command
• Add general tips: /metrics endpoint, CDN blocking, never delete apps

Deployment (4-deployment.md):

• Add port binding guidance
• Add "never delete-recreate" warning with clean stale files workflow
• Add resource attachment via API PATCH (critical — without it, gateway shows "App Not Available")
• Add permissions configuration via CLI

Lakebase (5-lakebase.md):

• Add OAuth security label error fix (SECURITY LABEL FOR databricks_auth)
• Add PGPASSWORD empty workaround using SDK token generation
• Add Lakebase Sync (reverse ETL) quick-start instructions

Troubleshooting (SKILL.md):

• "App Not Available" after deploy
• Frontend loads blank/black (CDN blocking)
• React page crashes to blank (Error #310)
• Gateway health check fails

databricks-app-apx (1 file, +385B)

• Add React Error #310 (hooks ordering) troubleshooting entry

databricks-model-serving (1 file, +61B)

• Add system.ai.similarity_search to built-in UC functions table

Test plan

• Verify SKILL.md renders correctly in GitHub
• Verify valueFrom YAML syntax matches current Databricks Apps runtime
• Verify API PATCH endpoints are current (/api/2.0/apps/<app-name>)
• Confirm system.ai.similarity_search is a valid built-in function

[CheeYuTan]

Test Results

Test	Status	Details
CI Validation (validate_skills.py)	PASS	All 26 skills validated
SKILL.md Frontmatter	PASS	All 3 modified skills (databricks-app-python, databricks-app-apx, databricks-model-serving) have valid frontmatter
No /metrics endpoint	PASS	Zero matches across all files — incorrect requirement properly excluded
OAuth patterns	PASS	Uses correct from databricks.sdk import WorkspaceClient pattern
install_skills.sh	PASS	No changes needed

Changes Summary (7 files, +173/-23 lines)

File	What Changed
databricks-app-apx/SKILL.md	Added React Error #310 troubleshooting (hooks after early returns)
databricks-app-python/SKILL.md	4 new critical rules (no CDN, no delete-recreate, no dev files, hooks ordering). 3 new troubleshooting entries
databricks-app-python/2-app-resources.md	Fixed valueFrom syntax to use nested resource: key. Added CLI API PATCH for resource attachment
databricks-app-python/3-frameworks.md	Port-binding notes for Streamlit/Flask/FastAPI. CDN blocking warning
databricks-app-python/4-deployment.md	Fixed valueFrom syntax. Rewrote redeployment (clean stale files, not delete-all). Added permissions config via CLI
databricks-app-python/5-lakebase.md	OAuth/SP troubleshooting. generate_database_credential SDK pattern. Lakebase Sync (reverse ETL) section
databricks-model-serving/4-tools-integration.md	Added system.ai.similarity_search to built-in UC functions

All changes are battle-tested fixes from real deployments.