feat: add code-simplifier power

[Raishin]

## Summary

Adds a new `code-simplifier` Kiro power for behavior-preserving refactoring guidance across multiple languages and frameworks.

This power is structured as a Guided MCP Power with:
- `POWER.md` for activation, onboarding, workflows, and troubleshooting
- `mcp.json` for Context7 integration
- `steering/` guides for language-specific and framework-specific simplification patterns

## What Changed

- Added `code-simplifier/POWER.md`
- Added `code-simplifier/mcp.json` with Context7 MCP configuration
- Added language and framework steering guides for:
  - C
  - C#
  - Java
  - Spring Boot
  - JavaScript
  - Kotlin
  - PHP
  - Laravel
  - Symfony
  - Python
  - Django
  - FastAPI
  - FastMCP
  - Flask
  - Rust
  - TypeScript
- Added `code-simplifier` to the repository catalog in `README.md`

## Why

This fills a gap in the powers catalog for maintainability-focused refactoring support.

The power is designed to help Kiro:
- simplify code without changing behavior
- load only the relevant steering for the current stack
- use Context7 when current framework or library conventions matter
- keep refactors narrow, readable, and aligned with repository conventions

## Review Guidance

Suggested review order:
1. `code-simplifier/POWER.md`
2. `code-simplifier/mcp.json`
3. `code-simplifier/steering/core.md`
4. Stack-specific steering files as needed
5. `README.md`

Focus areas:
- Kiro power format consistency
- MCP setup and tool naming accuracy
- Steering file scope boundaries
- Activation clarity and reviewer usability

## Testing

Tested in Kiro AI IDE.

Validated:
- local power installation
- activation behavior
- steering file usage
- power structure consistency with the existing repository format

## Notes

- This PR adds a new power only; it does not modify existing powers.
- Context7 remains optional at runtime for deeper docs lookup, but the steering guidance is usable on its own.