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/frontmatter.ts

@@ -0,0 +1,34 @@
+export interface FrontmatterResult<T = Record<string, string>> {
+  data: T
+  body: string
+}
+
+export function parseFrontmatter<T = Record<string, string>>(
+  content: string
+): FrontmatterResult<T> {
+  const frontmatterRegex = /^---\r?\n([\s\S]*?)\r?\n---\r?\n([\s\S]*)$/
+  const match = content.match(frontmatterRegex)
+
+  if (!match) {
+    return { data: {} as T, body: content }
+  }
+
+  const yamlContent = match[1]
+  const body = match[2]
+
+  const data: Record<string, string | boolean> = {}
+  for (const line of yamlContent.split("\n")) {
+    const colonIndex = line.indexOf(":")
+    if (colonIndex !== -1) {
+      const key = line.slice(0, colonIndex).trim()
+      let value: string | boolean = line.slice(colonIndex + 1).trim()
+
+      if (value === "true") value = true
+      else if (value === "false") value = false
+
+      data[key] = value
+    }
+  }
+
+  return { data: data as T, body }
+}