feat(coding-agent): make skill invocation messages collapsible

- Add ParsedSkillBlock interface and parseSkillBlock() function
- Change skill expansion to use XML-style <skill> tags
- Add SkillInvocationMessageComponent for collapsible display
- Collapsed: single line with skill name and expand hint
- User message rendered separately after skill block

Fixes #894

packages/coding-agent/src/core/agent-session.ts

@@ -70,6 +70,33 @@ import { buildSystemPrompt } from "./system-prompt.js";
 import type { BashOperations } from "./tools/bash.js";
 import { createAllTools } from "./tools/index.js";
 
+// ============================================================================
+// Skill Block Parsing
+// ============================================================================
+
+/** Parsed skill block from a user message */
+export interface ParsedSkillBlock {
+	name: string;
+	location: string;
+	content: string;
+	userMessage: string | undefined;
+}
+
+/**
+ * Parse a skill block from message text.
+ * Returns null if the text doesn't contain a skill block.
+ */
+export function parseSkillBlock(text: string): ParsedSkillBlock | null {
+	const match = text.match(/^<skill name="([^"]+)" location="([^"]+)">\n([\s\S]*?)\n<\/skill>(?:\n\n([\s\S]+))?$/);
+	if (!match) return null;
+	return {
+		name: match[1],
+		location: match[2],
+		content: match[3],
+		userMessage: match[4]?.trim() || undefined,
+	};
+}
+
 /** Session-specific events that extend the core AgentEvent */
 export type AgentSessionEvent =
 	| AgentEvent
@@ -796,9 +823,8 @@ export class AgentSession {
 		try {
 			const content = readFileSync(skill.filePath, "utf-8");
 			const body = stripFrontmatter(content).trim();
-			const header = `Skill location: ${skill.filePath}\nReferences are relative to ${skill.baseDir}.`;
-			const skillMessage = `${header}\n\n${body}`;
-			return args ? `${skillMessage}\n\n---\n\nUser: ${args}` : skillMessage;
+			const skillBlock = `<skill name="${skill.name}" location="${skill.filePath}">\nReferences are relative to ${skill.baseDir}.\n\n${body}\n</skill>`;
+			return args ? `${skillBlock}\n\n${args}` : skillBlock;
 		} catch (err) {
 			// Emit error like extension commands do
 			this._extensionRunner?.emitError({