feat(dbops): add default-authoring-instance MCP resource

[vivonk]

Summary

• Adds database_default_authoring_instance resource to the dbops MCP toolset
• Maps to GET /dbops/v1/orgs/{org}/projects/{project}/dbschema/{dbschema}/default-authoring-instance
• Returns the best instance for LLM change authoring (oldest with snapshot, fallback to oldest overall)
• Backend API added in db-devops-service PR

Test plan

• npx tsc --noEmit passes (verified locally)
• MCP Inspector: harness_get(resource_type="database_default_authoring_instance", dbschema_id="<id>") returns expected response
• 404 returned when schema has no instances

🤖 Generated with Claude Code

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