feat(skills): support extends: bundled in SKILL.md to allow extending bundled skills

[wenshao]

Summary

The bundled /review skill ships with 4 fixed review dimensions (Correctness & Security, Code Quality, Performance & Efficiency, Undirected Audit). Users who want to add custom review dimensions (e.g., Accessibility, API Compatibility, Compliance) currently have to copy the entire SKILL.md into .qwen/skills/review/SKILL.md and maintain it themselves — a full override with no composition.

This proposal adds a generic extends: bundled mechanism to the skill system, allowing users to append content to a bundled skill without replacing it entirely.

Motivation

Different teams care about different review aspects:

• Frontend teams may want an Accessibility dimension
• API teams may want an API Compatibility dimension
• Regulated industries may want a Compliance & Privacy dimension

The current skill precedence system (project > user > extension > bundled) only supports full replacement. This forces users to fork the bundled SKILL.md, losing future updates. A composition mechanism solves this cleanly.

Design

User experience

Create .qwen/skills/review/SKILL.md:

---
name: review
extends: bundled
---

### Agent 5: Accessibility

Focus areas:

- ARIA attributes and roles on interactive elements
- Keyboard navigation support
- Screen reader compatibility
- Color contrast ratios

### Agent 6: API Compatibility

Focus areas:

- Breaking changes to public APIs
- Backward compatibility of data formats

When /review is invoked, the user's body is appended to the bundled skill's body. The LLM sees all 6 dimensions and launches 6 parallel review agents.

Implementation

1. SkillConfig type (packages/core/src/skills/types.ts)

Add optional field:

extends?: SkillLevel;

2. SkillManager.loadSkill (packages/core/src/skills/skill-manager.ts)

When a skill has extends: bundled:

• Load the bundled skill with the same name
• Concatenate: bundledSkill.body + "\n\n" + extendingSkill.body
• Return the merged result

if (skill.extends === 'bundled') {
  const base = await this.findSkillByNameAtLevel(skill.name, 'bundled');
  if (!base) {
    throw new SkillError(`Cannot extend: bundled skill "${skill.name}" not found`, SkillErrorCode.NOT_FOUND);
  }
  skill.body = base.body + '\n\n' + skill.body;
  skill.extends = undefined; // mark as resolved
}

3. SKILL.md wording (packages/core/src/skills/bundled/review/SKILL.md)

-Launch **four parallel review agents** to analyze the changes from different angles.
+Launch **parallel review agents** (one per dimension listed below) to analyze the changes from different angles.

-Combine results from all four agents into a single, well-organized review.
+Combine results from all agents into a single, well-organized review.

What this does NOT change

• BundledSkillLoader — remains generic, no skill-specific logic
• settings.json — no new settings
• Directory conventions — reuses existing .qwen/skills/
• File watching — .qwen/skills/ is already watched

Edge cases

Scenario	Behavior
extends: bundled but no bundled skill with that name	Error: Cannot extend: bundled skill "xxx" not found
No extends field	Current behavior (full override), no breaking change
extends with a value other than bundled	Error: unsupported (v1 only supports bundled)
Extending skill has empty body	Use bundled body as-is, emit warning
User wants to disable a built-in dimension	Add natural language instruction in appended body (e.g., "**Skip the Undirected Audit dimension
above.**") — the review is LLM-driven, so this works

Scope

• packages/core/src/skills/types.ts — add extends field
• packages/core/src/skills/skill-manager.ts — extend resolution logic (~20 lines)
• packages/core/src/skills/bundled/review/SKILL.md — dynamic agent count wording
• Tests for the above

[Br1an67]

I've implemented this — PR incoming with the extends: bundled mechanism as described in the issue.