feat: update skill frontmatter to MCP skills-as-groups spec format

Replace allowed-tools YAML array with metadata.io.modelcontextprotocol/tools
space-separated string in skill resource frontmatter, aligning with the
proposed MCP skills-as-groups specification.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums

pkg/github/skill_resources.go

@@ -496,10 +496,8 @@ func buildSkillContent(skill skillDefinition) string {
 	b.WriteString("---\n")
 	fmt.Fprintf(&b, "name: %s\n", skill.name)
 	fmt.Fprintf(&b, "description: %s\n", skill.description)
-	b.WriteString("allowed-tools:\n")
-	for _, tool := range skill.allowedTools {
-		fmt.Fprintf(&b, "  - %s\n", tool)
-	}
+	b.WriteString("metadata:\n")
+	fmt.Fprintf(&b, "  io.modelcontextprotocol/tools: \"%s\"\n", strings.Join(skill.allowedTools, " "))
 	b.WriteString("---\n\n")
 	b.WriteString(skill.body)
 	return b.String()

pkg/github/skill_resources_test.go

@@ -42,8 +42,8 @@ func TestBuildSkillContent(t *testing.T) {
 	assert.Contains(t, content, "---\n")
 	assert.Contains(t, content, "name: test-skill\n")
 	assert.Contains(t, content, "description: A test skill\n")
-	assert.Contains(t, content, "  - tool_a\n")
-	assert.Contains(t, content, "  - tool_b\n")
+	assert.Contains(t, content, "metadata:\n")
+	assert.Contains(t, content, "  io.modelcontextprotocol/tools: \"tool_a tool_b\"\n")
 	assert.Contains(t, content, "# Test\n")
 }