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/examples/sdk/04-skills.ts

@@ -14,6 +14,7 @@ const customSkill: Skill = {
 	filePath: "/virtual/SKILL.md",
 	baseDir: "/virtual",
 	source: "path",
+	disableModelInvocation: false,
 };
 
 const loader = new DefaultResourceLoader({