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/go/SKILL.md

@@ -0,0 +1,22 @@
+---
+name: go
+description: Idiomatic Go conventions. Use when editing .go files or go.mod/go.sum.
+---
+
+# Go Conventions
+
+Write strictly idiomatic Go following standard formatting and best practices.
+
+## Formatting and style
+- `gofmt` / `goimports` formatting is mandatory.
+- Follow [Effective Go](https://go.dev/doc/effective_go) and the [Go Code Review Comments](https://github.com/golang/go/wiki/CodeReviewComments).
+
+## Design
+- Accept `context.Context` as the first parameter on functions doing I/O or long work; honor cancellation.
+- Compose via small interfaces; accept interfaces, return structs.
+- Wire dependencies explicitly via constructor injection; avoid package-level mutable globals and DI frameworks.
+- Return errors explicitly; wrap with `%w` (`fmt.Errorf("do thing: %w", err)`); classify with `errors.Is` / `errors.As`; never discard errors.
+
+## Testing
+- Write table-driven tests in `_test.go` using the standard `testing` package.
+- Use `t.Run` for subtests; report failures with `t.Errorf` / `t.Fatalf`.