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/bash/SKILL.md

@@ -0,0 +1,21 @@
+---
+name: bash
+description: Shell scripting conventions (set -euo pipefail, quoting, shellcheck). Use when editing .sh or .bash files.
+---
+
+# Bash Conventions
+
+## Robustness
+- Begin scripts with `#!/usr/bin/env bash` and `set -euo pipefail`.
+- Pass `shellcheck` cleanly; treat its warnings as bugs.
+
+## Quoting and expansion
+- Double-quote all variable expansions (`"$var"`, `"${array[@]}"`).
+- Prefer `[[ ]]` over `[ ]`; use arrays for argument lists rather than space-separated strings.
+- Avoid `eval`; do not parse `ls` output.
+
+## Structure
+- Declare function-local variables with `local`.
+- Use `trap` to clean up temporary files and directories.
+- Use `printf` over `echo` for predictable output.
+- Prefer a `$TMPDIR`-based path over hardcoded `/tmp` literals.