Add new databricks-agent-to-app skill for serving-to-app migration

[CheeYuTan]

Summary

• New skill addressing issue Skill Suggestion: Convert existing agents on model serving to apps implementation #109: convert agents deployed on Model Serving endpoints into Databricks Apps with a web UI
• Includes step-by-step migration guide, Streamlit and Gradio chat app templates, app.yaml configuration, multi-turn conversation support, OAuth authentication, and 8-entry Common Issues table
• Registered in install_skills.sh

Closes #109

Test proof

Tested agent-to-app migration patterns against live workspace e2-demo-field-eng:

Test	Result
List serving endpoints (agent discovery)	PASS — found advait_gpt5 (READY), AFIPDF (READY), etc.
Get endpoint details	PASS — full ServingEndpointDetailed object returned
SDK Config import (used in app auth)	PASS — databricks.sdk.config.Config importable
Get current user (for app personalization)	PASS — Steven Tan (steven.tan@databricks.com)
Serving endpoint query format verification	PASS — name + messages format confirmed

TEST: List serving endpoints
  PASS
  → [('advait_gpt5', READY), ('AFIPDF', READY), ('agent-auto-refresh', READY)]

TEST: Get endpoint details: advait_gpt5
  PASS
  → ServingEndpointDetailed(ai_gateway=AiGatewayConfig(...), name='advait_gpt5', ...)

TEST: SDK Config import for app auth
  PASS
  → <class 'databricks.sdk.config.Config'>

TEST: Get current user (for app personalization)
  PASS
  → Steven Tan (steven.tan@databricks.com)

5/5 core pattern tests passed. (Streamlit/Gradio import tests skipped — these are app-side dependencies installed via requirements.txt, not local dev dependencies.)