feat(skills): add bundled /review skill for code review

[wenshao]

Summary

Add a built-in /review bundled skill to qwen-code, providing out-of-the-box code review capability. Users can invoke /review to review local changes,
PRs, or specific files without any configuration.

Motivation

Currently qwen-code has no built-in code review skill. Users must manually configure project-level commands (.qwen/commands/) or skills
(.qwen/skills/) to get review functionality. Code review is a high-frequency workflow that should work out of the box.

Claude Code ships /review as a built-in feature available immediately after installation. qwen-code should have the same capability.

Implementation

1. New skill level: bundled

Add a bundled level to SkillLevel type, with the lowest precedence:

project > user > extension > bundled

Bundled skills are stored in packages/core/src/skills/bundled/ and shipped with the package. The .md files are automatically copied to dist/ during
build by the existing copy_files.js script. Users can override any bundled skill by creating a same-named skill at project/user level.

2. /review skill

Supports three modes:

Mode	Usage	Source
Local changes	/review	git diff + git diff --staged
PR review	/review 123	gh pr diff <number>
File review	/review src/foo.ts	git diff HEAD -- <file>

Spawns 4 parallel review agents:

Agent	Dimension	Focus
Agent 1	Correctness & Security	Logic errors, edge cases, vulnerabilities, type safety
Agent 2	Code Quality	Style consistency, naming, duplication, dead code
Agent 3	Performance & Efficiency	Bottlenecks, memory leaks, algorithmic complexity
Agent 4	Undirected Audit	No preset dimension — reviews code with a fresh perspective to catch issues the other three agents may miss (e.g.,
business logic soundness, boundary interactions, implicit assumptions)

Output format:

• Critical — Must fix before merging
• Suggestion — Recommended improvement
• Nice to have — Optional optimization

Final verdict: Approve / Request changes / Comment

Files changed

• packages/core/src/skills/types.ts — Add 'bundled' to SkillLevel
• packages/core/src/skills/skill-manager.ts — Load bundled skills from packages/core/src/skills/bundled/, resolve via import.meta.url
• packages/core/src/skills/bundled/review/SKILL.md — The review skill definition (new)