feat(opencode): add language-specific coding skills

- add conventions for Bash, C#, Go, Python, and TypeScript
- add frontend guidance for HTMX, PrimeVue, and Vue
- document formatting, typing, testing, async, and framework-specific practices
- reference official documentation for framework API details

.config/opencode/skills/csharp/SKILL.md

@@ -0,0 +1,37 @@
+---
+name: csharp
+description: C# and .NET coding conventions. Use when reading, writing, or reviewing .cs, .razor, .csproj, or .sln files, or any .NET (C#) code.
+---
+
+# C# / .NET Conventions
+
+Follow the Microsoft [Common C# code conventions](https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/coding-style/coding-conventions) and [.NET naming guidelines](https://learn.microsoft.com/en-us/dotnet/standard/design-guidelines/naming-guidelines). Match patterns already established in the target codebase.
+
+## Structure
+- Use file-scoped namespaces (`namespace Example.Services;`).
+- Nullable reference types are enabled solution-wide; do not suppress warnings without cause.
+- Use `record` types for DTOs and data carriers.
+- Use `required` / `init`-only properties for immutable domain models.
+- Use primary constructors for services and repositories that take injected dependencies.
+- Mark classes `sealed` by default unless inheritance is intended.
+- Prefix interfaces with `I` (`IRenderer`, `IRepository`).
+
+## Expressions
+- Use raw string literals (`""" ... """`) for multi-line SQL.
+- Use `[GeneratedRegex]` partial methods for regex.
+- Use collection expressions (`[]`, `[a, b]`).
+
+## Async and cancellation
+- Every async method takes `CancellationToken ct = default` and honors it cooperatively (`ct.ThrowIfCancellationRequested()` in long loops).
+- Never drop the token when calling downstream async APIs.
+
+## Logging
+- Inject `ILogger<T>` and use structured logging with named parameters:
+  `_logger.LogInformation("Generated {Count} transactions", count);`
+
+## Naming
+- `PascalCase` for types and members.
+- `_camelCase` for private fields.
+
+## Testing
+- Use xUnit. Mirror the source structure under a `Tests/` directory with a `Tests` suffix on file names.