fix(api): restore accidentally deleted agent-skills coverage route

Commit a5212536c2 (DeepAI provider feature) deleted
src/app/api/agent-skills/coverage/route.ts by mistake while touching
unrelated files, breaking tests/unit/agentSkills-routes.test.ts
(ERR_MODULE_NOT_FOUND) and the openapi-routes doc-sync gate, which
still documents GET /api/agent-skills/coverage.

src/app/api/agent-skills/coverage/route.ts

@@ -0,0 +1,25 @@
+/**
+ * GET /api/agent-skills/coverage
+ *
+ * Returns SkillCoverage: how many skills have a SKILL.md on filesystem vs catalog total.
+ *
+ * Response: SkillCoverage
+ */
+import { NextResponse } from "next/server";
+
+import { buildErrorBody } from "@omniroute/open-sse/utils/error.ts";
+import { computeCoverage } from "@/lib/agentSkills/catalog";
+
+export const dynamic = "force-dynamic";
+
+export async function GET() {
+  try {
+    const coverage = computeCoverage();
+    return NextResponse.json(coverage);
+  } catch (error) {
+    console.error("[API] GET /api/agent-skills/coverage error:", error);
+    return NextResponse.json(buildErrorBody(500, "Failed to compute skill coverage"), {
+      status: 500,
+    });
+  }
+}