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/core/system-prompt.ts

@@ -149,7 +149,7 @@ export function buildSystemPrompt(options: BuildSystemPromptOptions = {}): strin
 		// Append skills section (only if read tool is available)
 		const customPromptHasRead = !selectedTools || selectedTools.includes("read");
 		if (skillsEnabled && customPromptHasRead) {
-			const skills = loadSkills();
+			const { skills } = loadSkills();
 			prompt += formatSkillsForPrompt(skills);
 		}
 
@@ -255,7 +255,7 @@ Documentation:
 
 	// Append skills section (only if read tool is available)
 	if (skillsEnabled && hasRead) {
-		const skills = loadSkills();
+		const { skills } = loadSkills();
 		prompt += formatSkillsForPrompt(skills);
 	}