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/index.ts

@@ -0,0 +1,4 @@
+export * from "./frontmatter"
+export * from "./command-executor"
+export * from "./file-reference-resolver"
+export * from "./model-sanitizer"