feat: add /create-test command and plugin

[Fabien83560]

Introduces a new Claude Code plugin that automatically generates unit test files from source code analysis.

• plugin with manifest, command, and two specialized agents
• agent: extracts exports, parameters, return types, async behavior, external dependencies, and edge case hints
• agent: produces framework-specific test files with mocked dependencies, grouped test suites, and TODO-guided placeholders
• Command orchestrates a 9-phase workflow: argument parsing, file validation, framework detection (Jest/Vitest/Mocha/pytest/go-test), test path resolution, source analysis, test generation, optional preview, and file write with --force guard
• Root-level for immediate project use
• Supports TypeScript, JavaScript, Python, and Go