feat(shared): add shared utilities for command and skill loading

- frontmatter.ts: YAML frontmatter parser
- file-reference-resolver.ts: resolve @file references in markdown
- command-executor.ts: execute shell commands in markdown
- model-sanitizer.ts: sanitize model names for OpenCode compatibility

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/shared/model-sanitizer.ts

@@ -0,0 +1,13 @@
+/**
+ * Sanitizes model field from frontmatter.
+ * Always returns undefined to let SDK use default model.
+ *
+ * Claude Code and OpenCode use different model ID formats,
+ * so we ignore the model field and let OpenCode use its configured default.
+ *
+ * @param _model - Raw model value from frontmatter (ignored)
+ * @returns Always undefined to inherit default model
+ */
+export function sanitizeModelField(_model: unknown): undefined {
+  return undefined
+}