feat(a2a): add list-capabilities skill with markdown table of 42 skills

Implements executeListCapabilities() which calls getCatalog() + computeCoverage()
from the agentSkills catalog (F1/F2) and returns a markdown table covering all
42 skills (22 API + 20 CLI) with ID, name, category, area, endpoints/commands,
and raw SKILL.md URL, matching the §3.7 result contract.

src/lib/a2a/skills/listCapabilities.ts

@@ -0,0 +1,72 @@
+/**
+ * A2A Skill: List Capabilities
+ *
+ * Returns the full catalog of 42 OmniRoute agent skills (22 API + 20 CLI)
+ * as a markdown table with raw SKILL.md URLs for orchestrating agents.
+ */
+
+import type { A2ATask, TaskArtifact } from "../taskManager";
+import { getCatalog, computeCoverage } from "@/lib/agentSkills/catalog";
+import type { AgentSkill } from "@/lib/agentSkills/types";
+
+export interface ListCapabilitiesResult {
+  artifacts: TaskArtifact[];
+  metadata: {
+    coverage: {
+      api: { have: number; total: 22 };
+      cli: { have: number; total: 20 };
+    };
+    totalSkills: 42;
+    generatedAt: string;
+    source: "agent-skills-catalog";
+  };
+}
+
+function buildMarkdownTable(skills: AgentSkill[]): string {
+  const header = "| ID | Name | Category | Area | Endpoints/Commands | Raw URL |";
+  const separator = "| --- | --- | --- | --- | --- | --- |";
+
+  const rows = skills.map((skill) => {
+    const endpointsOrCommands =
+      skill.category === "api"
+        ? (skill.endpoints ?? []).join(", ") || "—"
+        : (skill.cliCommands ?? []).join(", ") || "—";
+
+    return `| ${skill.id} | ${skill.name} | ${skill.category} | ${skill.area} | ${endpointsOrCommands} | ${skill.rawUrl} |`;
+  });
+
+  return [header, separator, ...rows].join("\n");
+}
+
+export async function executeListCapabilities(_task: A2ATask): Promise<ListCapabilitiesResult> {
+  const catalog = getCatalog();
+  const coverage = computeCoverage();
+
+  const table = buildMarkdownTable(catalog);
+
+  const content = [
+    `# OmniRoute Agent Skills Catalog`,
+    ``,
+    `Total: ${catalog.length} skills (${coverage.api.total} API + ${coverage.cli.total} CLI)`,
+    ``,
+    table,
+  ].join("\n");
+
+  return {
+    artifacts: [
+      {
+        type: "text",
+        content,
+      },
+    ],
+    metadata: {
+      coverage: {
+        api: { have: coverage.api.have, total: 22 },
+        cli: { have: coverage.cli.have, total: 20 },
+      },
+      totalSkills: 42,
+      generatedAt: coverage.generatedAt,
+      source: "agent-skills-catalog",
+    },
+  };
+}