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

[Br1an67]

TLDR

Closes #2379.

Adds an extends: bundled mechanism to the skill system, allowing users to append content to a bundled skill without replacing it entirely.

Dive Deeper

Problem

Users who want to add custom review dimensions (e.g., Accessibility, API Compatibility) currently have to copy the entire bundled SKILL.md into .qwen/skills/review/SKILL.md and maintain it — a full override with no composition. Future updates to the bundled skill are lost.

Solution

A new optional extends: bundled field in SKILL.md frontmatter. When set, the extending skill's body is appended to the bundled skill's body during loading.

Example (.qwen/skills/review/SKILL.md):

---
name: review
description: Extended code review
extends: bundled
---

### Agent 5: Accessibility
Focus areas: ARIA attributes, keyboard navigation, screen reader compatibility

When /review is invoked, the user sees the bundled content plus the custom dimensions.

Changes

• types.ts: Added optional extends?: 'bundled' field to SkillConfig
• skill-manager.ts:
  • Parse extends from YAML frontmatter (with validation — only 'bundled' is accepted)
  • Added resolveExtends() that loads the bundled base skill and concatenates bodies
  • All loadSkill paths call resolveExtends() before returning
• skill-manager.test.ts: 4 new tests covering parsing, merging, error handling

What this does NOT change

• Directory conventions
• settings.json schema
• Bundled skill loading logic
• Existing skill precedence (project > user > extension > bundled)

Testing Matrix

Scenario	Expected	Status
Parse extends: bundled from frontmatter	config.extends === 'bundled'	✅
Reject extends: user (invalid value)	Throws descriptive error	✅
Project skill with extends: bundled	Bodies merged, extends cleared	✅
extends: bundled referencing non-existent bundled skill	Throws SkillError NOT_FOUND	✅
Existing tests (40)	All pass unchanged	✅