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/skill-content.ts

@@ -0,0 +1,29 @@
+import { createBuiltinSkills } from "../builtin-skills/skills"
+
+export function resolveSkillContent(skillName: string): string | null {
+	const skills = createBuiltinSkills()
+	const skill = skills.find((s) => s.name === skillName)
+	return skill?.template ?? null
+}
+
+export function resolveMultipleSkills(skillNames: string[]): {
+	resolved: Map<string, string>
+	notFound: string[]
+} {
+	const skills = createBuiltinSkills()
+	const skillMap = new Map(skills.map((s) => [s.name, s.template]))
+
+	const resolved = new Map<string, string>()
+	const notFound: string[] = []
+
+	for (const name of skillNames) {
+		const template = skillMap.get(name)
+		if (template) {
+			resolved.set(name, template)
+		} else {
+			notFound.push(name)
+		}
+	}
+
+	return { resolved, notFound }
+}