fix(agent-skills): use res.text() for text/markdown endpoint (GAP-1 critical bug)

The /api/agent-skills/[id]/raw endpoint returns text/markdown (plain string),
not a JSON envelope. The client was incorrectly calling res.json() and
unpacking a non-existent `body` field, causing all preview panes to show
empty content. Switch to res.text() and update the test mock to match.

src/app/(dashboard)/dashboard/agent-skills/AgentSkillsPageClient.tsx

@@ -95,8 +95,8 @@ export function AgentSkillsPageClient(): JSX.Element {
       try {
         const res = await fetch(`/api/agent-skills/${id}/raw`);
         if (!res.ok) throw new Error(`HTTP ${res.status}`);
-        const json = (await res.json()) as { body: string };
-        setMarkdownCache((prev) => new Map(prev).set(id, json.body ?? ""));
+        const body = await res.text();
+        setMarkdownCache((prev) => new Map(prev).set(id, body));
       } catch {
         setMarkdownCache((prev) => {
           const next = new Map(prev);

tests/unit/agent-skills-page.test.tsx

@@ -106,9 +106,9 @@ function mockFetch(skills: AgentSkill[], coverage: SkillCoverage, rawMarkdown =
       });
     }
     if (/\/api\/agent-skills\/.+\/raw/.test(urlStr)) {
-      return new Response(JSON.stringify({ body: rawMarkdown }), {
+      return new Response(rawMarkdown, {
         status: 200,
-        headers: { "Content-Type": "application/json" },
+        headers: { "Content-Type": "text/markdown; charset=utf-8" },
       });
     }
     return new Response(JSON.stringify({}), { status: 404 });