Skills standard compliance

Implement Agent Skills standard (https://agentskills.io/specification):
- Validate name (must match parent dir, lowercase, max 64 chars)
- Validate description (required, max 1024 chars)
- Warn on unknown frontmatter fields
- Warn on name collisions (keep first)
- Change prompt format to XML structure
- Remove {baseDir} placeholder (use relative paths)
- Add tests and update documentation

fixes #231

packages/coding-agent/src/index.ts

@@ -81,10 +81,12 @@ export {
 export {
 	formatSkillsForPrompt,
 	type LoadSkillsFromDirOptions,
+	type LoadSkillsResult,
 	loadSkills,
 	loadSkillsFromDir,
 	type Skill,
 	type SkillFrontmatter,
+	type SkillWarning,
 } from "./core/skills.js";
 // Tools
 export { bashTool, codingTools, editTool, readTool, writeTool } from "./core/tools/index.js";