feat(coding-agent): support disable-model-invocation frontmatter for skills

When set to true, the skill is hidden from the system prompt, preventing
agentic invocation. Users can still invoke explicitly via /skill:name.

Also fixes pre-existing test bug where source expectation was wrong.

Fixes #927

packages/coding-agent/test/resource-loader.test.ts

@@ -155,6 +155,7 @@ Content`,
 				filePath: "/fake/path",
 				baseDir: "/fake",
 				source: "custom",
+				disableModelInvocation: false,
 			};
 			const loader = new DefaultResourceLoader({
 				cwd,