chore(backend): install AI agent skills for the .NET stack

Run autoskills in backend/: detects .NET/C#/ASP.NET Core/Minimal API and
installs 13 skills for universal (.agents) and claude-code (.claude) agents,
with backend/skills-lock.json. Adds ASP.NET Core, Minimal API + OpenAPI,
file-upload and containerization skills on top of the prior C#/.NET set.
Removes the stale root skills from the old layout.

backend/.agents/skills/aspnet-core/SKILL.md

@@ -0,0 +1,61 @@
+---
+name: aspnet-core
+description: Build, review, refactor, or architect ASP.NET Core web applications using current official guidance for .NET web development. Use when working on Blazor Web Apps, Razor Pages, MVC, Minimal APIs, controller-based Web APIs, SignalR, gRPC, middleware, dependency injection, configuration, authentication, authorization, testing, performance, deployment, or ASP.NET Core upgrades.
+---
+
+# ASP.NET Core
+
+## Overview
+
+Choose the right ASP.NET Core application model, compose the host and request pipeline correctly, and implement features in the framework style Microsoft documents today.
+
+Load the smallest set of references that fits the task. Do not load every reference by default.
+
+## Workflow
+
+1. Confirm the target framework, SDK, and current app model.
+2. Open [references/stack-selection.md](references/stack-selection.md) first for new apps or major refactors.
+3. Open [references/program-and-pipeline.md](references/program-and-pipeline.md) next for `Program.cs`, DI, configuration, middleware, routing, logging, and static assets.
+4. Open exactly one primary app-model reference:
+   - [references/ui-blazor.md](references/ui-blazor.md)
+   - [references/ui-razor-pages.md](references/ui-razor-pages.md)
+   - [references/ui-mvc.md](references/ui-mvc.md)
+   - [references/apis-minimal-and-controllers.md](references/apis-minimal-and-controllers.md)
+5. Add cross-cutting references only as needed:
+   - [references/data-state-and-services.md](references/data-state-and-services.md)
+   - [references/security-and-identity.md](references/security-and-identity.md)
+   - [references/realtime-grpc-and-background-work.md](references/realtime-grpc-and-background-work.md)
+   - [references/testing-performance-and-operations.md](references/testing-performance-and-operations.md)
+6. Open [references/versioning-and-upgrades.md](references/versioning-and-upgrades.md) before introducing new platform APIs into an older solution or when migrating between major versions.
+7. Use [references/source-map.md](references/source-map.md) when you need the Microsoft Learn section that corresponds to a task not already covered by the focused references.
+
+## Default Operating Assumptions
+
+- Prefer the latest stable ASP.NET Core and .NET unless the repository or user request pins an older target.
+- As of March 2026, prefer .NET 10 / ASP.NET Core 10 for new production work. Treat ASP.NET Core 11 as preview unless the user explicitly asks for preview features.
+- Prefer `WebApplicationBuilder` and `WebApplication`. Avoid older `Startup` and `WebHost` patterns unless the codebase already uses them or the task is migration.
+- Prefer built-in DI, options/configuration, logging, ProblemDetails, OpenAPI, health checks, rate limiting, output caching, and Identity before adding third-party infrastructure.
+- Keep feature slices cohesive so the page, component, endpoint, controller, validation, service, data access, and tests are easy to trace.
+- Respect the existing app model. Do not rewrite Razor Pages to MVC or controllers to Minimal APIs without a clear reason.
+
+## Reference Guide
+
+- [references/_sections.md](references/_sections.md): Quick index and reading order.
+- [references/stack-selection.md](references/stack-selection.md): Choose the right ASP.NET Core application model and template.
+- [references/program-and-pipeline.md](references/program-and-pipeline.md): Structure `Program.cs`, services, middleware, routing, configuration, logging, and static assets.
+- [references/ui-blazor.md](references/ui-blazor.md): Build Blazor Web Apps, choose render modes, and use components, forms, and JS interop correctly.
+- [references/ui-razor-pages.md](references/ui-razor-pages.md): Build page-focused server-rendered apps with handlers, model binding, and conventions.
+- [references/ui-mvc.md](references/ui-mvc.md): Build controller/view applications with clear separation of concerns.
+- [references/apis-minimal-and-controllers.md](references/apis-minimal-and-controllers.md): Build HTTP APIs with Minimal APIs or controllers, including validation and response patterns.
+- [references/data-state-and-services.md](references/data-state-and-services.md): Use EF Core, `DbContext`, options, `IHttpClientFactory`, session, temp data, and app state responsibly.
+- [references/security-and-identity.md](references/security-and-identity.md): Apply authentication, authorization, Identity, secrets, data protection, CORS, CSRF, and HTTPS guidance.
+- [references/realtime-grpc-and-background-work.md](references/realtime-grpc-and-background-work.md): Use SignalR, gRPC, and hosted services.
+- [references/testing-performance-and-operations.md](references/testing-performance-and-operations.md): Add integration tests, browser tests, caching, compression, health checks, rate limits, and deployment concerns.
+- [references/versioning-and-upgrades.md](references/versioning-and-upgrades.md): Handle target frameworks, breaking changes, obsolete APIs, and migrations.
+- [references/source-map.md](references/source-map.md): Map the official ASP.NET Core documentation tree to the references in this skill.
+
+## Execution Notes
+
+- When generating new code, start from the correct `dotnet new` template and keep the generated structure recognizable.
+- When editing an existing solution, follow the solution's conventions first and use these references to avoid framework misuse or outdated patterns.
+- When a task mentions "latest", verify the feature on Microsoft Learn or the ASP.NET Core docs repo before relying on memory.