feat(skill-loader): add skill-content resolver for agent skills

Add resolveMultipleSkills() for resolving skill content to prepend to agent prompts.
Includes test coverage for resolution logic.

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

src/features/opencode-skill-loader/index.ts

@@ -1,3 +1,4 @@
 export * from "./types"
 export * from "./loader"
 export * from "./merger"
+export * from "./skill-content"