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/htmx/SKILL.md

@@ -0,0 +1,22 @@
+---
+name: htmx
+description: HTMX v2 progressive-enhancement and hypermedia patterns.
+---
+
+# HTMX Conventions
+
+Follow [HTMX](https://htmx.org) v2 and [Hypermedia Systems](https://hypermedia.systems/book/contents/) best practices.
+
+## Philosophy
+- Treat the server as the source of truth; return HTML fragments, not JSON, for hypermedia interactions.
+- Prefer `hx-*` attributes over client-side JavaScript; reach for client JavaScript only when htmx cannot express the interaction.
+- Build progressively: ensure pages function without JavaScript, then enhance with htmx.
+
+## Patterns
+- Target fragments with `hx-target` and swap with `hx-swap` (prefer `outerHTML` or `innerHTML` as appropriate).
+- Return partial templates or fragments from endpoints; treat partials as isolated components that receive data via the page model.
+- Use Out-of-Band Swaps (`hx-swap-oob`) to update multiple regions from a single response.
+- Handle errors with a 4xx/5xx partial or `hx-on::responseError`.
+
+## Keep clean
+- Return semantic HTML from endpoints; avoid mixing `hx-*` attributes with conflicting client-side handlers.