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/sdk-skills.test.ts

@@ -80,6 +80,7 @@ This is a test skill.
 			filePath: "/fake/path/SKILL.md",
 			baseDir: "/fake/path",
 			source: "custom" as const,
+			disableModelInvocation: false,
 		};
 
 		const resourceLoader: ResourceLoader = {