feat: add code-simplify skill and /simplify slash command

[roomote]

This PR adds a built-in code simplification skill and corresponding /simplify slash command based on the Claude code-simplifier plugin.

Changes

• New built-in skill: code-simplify - Simplifies and refines code for clarity, consistency, and maintainability while preserving functionality
• New slash command: /simplify - Convenient command to trigger the code-simplify skill
• Extended skill system: Updated SkillMetadata type to support "built-in" skills alongside "global" and "project" skills
• Skills manager improvements: Implemented built-in skills loading with proper override priority (project > global > built-in)
• Comprehensive tests: Added and updated tests for the new functionality

Usage

Users can now use /simplify to analyze and improve recently modified code, focusing on:

• Reducing unnecessary complexity
• Applying project-specific best practices
• Improving readability through clear naming
• Avoiding nested ternaries and overly compact code
• Maintaining explicit, understandable code

The skill operates autonomously and can be triggered via the /simplify slash command for convenience.

Testing

• All new tests pass
• Updated existing tests to account for built-in skills
• TypeScript type checking passes
• Linting passes

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Important

Adds code-simplify skill and /simplify command, updates skills management for built-in skills, and enhances tests.

• New Features:
  • Adds code-simplify skill in built-in-skills.ts for code clarity and maintainability.
  • Introduces /simplify command in built-in-commands.ts to trigger the code-simplify skill.
• Skills Management:
  • Updates SkillsManager in SkillsManager.ts to load built-in skills with priority: project > global > built-in.
  • Modifies SkillMetadata in skills.ts to include built-in as a source.
• Testing:
  • Adds tests in built-in-commands.spec.ts and SkillsManager.spec.ts for new command and skill.
  • Updates existing tests to account for built-in skills.

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[roomote]

Rooviewer   See task on Roo Cloud

Review completed. Found 1 minor issue to address.

• Circular dependency: built-in-skills.ts imports types from SkillsManager.ts instead of directly from shared/skills.ts

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

[roomote]

This import creates a circular dependency with SkillsManager.ts, which imports from this file. While it works at runtime because these are type-only imports (erased during compilation), it's a code smell that could cause issues if runtime values are ever added. Import directly from the shared types module instead.

Suggested change

	import { SkillMetadata, SkillContent } from "./SkillsManager"
	import { SkillMetadata, SkillContent } from "../../shared/skills"

_{Fix it with Roo Code or mention @roomote and request a fix.}