Skills: use native app resource and native postgres workflow

[dhruv0811]

Summary

Databricks CLI v0.298.0 ships native support for app-to-app bundle resources, and v0.295.0+ has native autoscaling postgres. Replace the manual workarounds in our skills/templates with the declarative YAML approach.

• add-tools-{langgraph,openai}: new app.yaml example + native snippet in SKILL.md and custom-mcp-server.md
• deploy/SKILL.md: drop post-deploy postgres API PATCH section; only the table-level permission grant remains
• agent-openai-agents-sdk-multiagent/databricks.yml: replace 17-line workaround comment with a 4-line native app resource block
• Sync skills across all templates

Before (app-to-app workaround)

databricks apps get <agent-app> --output json | jq -r '.service_principal_name'
databricks apps update-permissions <mcp-server-app> \
  --json '{"access_control_list": [{"service_principal_name": "<sp>", "permission_level": "CAN_USE"}]}'

After (native bundle resource)

resources:
  - name: 'mcp_server'
    app:
      name: 'mcp-my-server'
      permission: CAN_USE