chore: import .claude command skills, CLAUDE.md, .gitignore, next_steps.md

- Vendor local .claude/ command skills (autopilot, plan, implement, etc.)
- Add CLAUDE.md pointing slash commands to .claude/commands/*/SKILL.md
- Untrack docs-Lokal/ and ignore .planning/ for local-only planning docs
- Include next_steps.md pulled from upstream

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/rules/dotnet.mdc

@@ -0,0 +1,17 @@
+---
+description: ".NET/C# coding conventions: naming, async patterns, DI, EF Core, error handling, layered architecture"
+globs: ["**/*.cs", "**/*.csproj", "**/*.sln"]
+---
+# .NET / C#
+
+- PascalCase for classes, methods, properties, namespaces; camelCase for locals and parameters; prefix interfaces with `I`
+- Use `async`/`await` for I/O-bound operations, do not suffix async methods with Async
+- Use dependency injection via constructor injection; register services in `Program.cs`
+- Use linq2db for small projects, EF Core with migrations for big ones; avoid raw SQL unless performance-critical; prevent N+1 with `.Include()` or projection
+- Use `Result<T, E>` pattern or custom error types over throwing exceptions for expected failures
+- Use `var` when type is obvious; prefer LINQ/lambdas for collections
+- Use C# 10+ features: records for DTOs, pattern matching, null-coalescing
+- Layer structure: Controllers -> Services (interfaces) -> Repositories -> Data/EF contexts
+- Use Data Annotations or FluentValidation for input validation
+- Use middleware for cross-cutting: auth, error handling, logging
+- API versioning via URL or header; document with XML comments for Swagger/OpenAPI