Standardize route-owned create flows

[cwilhit]

Summary

• codify concise design-book guidance for route-owned creation of consequential resources
• add one shared create-page skeleton and apply it across MCP, Agents, App Studio, Code, Databricks, Edges, and Infrastructure where the task warrants a dedicated route
• preserve contextual creation for compact subordinate actions
• add readable provider-owned routes, lifecycle and authority fencing, cached-list refresh behavior, and cross-provider conformance coverage

Verification

• make verify-portalkit verify-ui-conformance
• Code portal: npm test, npm run typecheck, npm run build
• Edges portal: npm test, npm run typecheck, npm run build
• Databricks portal: npm test, npm run typecheck, npm run build
• git diff --check