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/modes/interactive/interactive-mode.ts

@@ -305,13 +305,20 @@ export class InteractiveMode {
 		}
 
 		// Show loaded skills
-		const skills = loadSkills();
+		const { skills, warnings: skillWarnings } = loadSkills();
 		if (skills.length > 0) {
 			const skillList = skills.map((s) => theme.fg("dim", `  ${s.filePath}`)).join("\n");
 			this.chatContainer.addChild(new Text(theme.fg("muted", "Loaded skills:\n") + skillList, 0, 0));
 			this.chatContainer.addChild(new Spacer(1));
 		}
 
+		// Show skill warnings if any
+		if (skillWarnings.length > 0) {
+			const warningList = skillWarnings.map((w) => theme.fg("warning", `  ${w.skillPath}: ${w.message}`)).join("\n");
+			this.chatContainer.addChild(new Text(theme.fg("warning", "Skill warnings:\n") + warningList, 0, 0));
+			this.chatContainer.addChild(new Spacer(1));
+		}
+
 		// Show loaded custom tools
 		if (this.customTools.size > 0) {
 			const toolList = Array.from(this.customTools.values())