feat: add StackGen infrastructure-as-code power

[sks]

What changes

• Adds a new StackGen power that connects Kiro to the StackGen MCP server, enabling AI-assisted cloud infrastructure management.

What's included

• stackgen/POWER.md — Power manifest and documentation covering 25+ tools across appstack management, resource operations, environment profiles, module management, Git integration, and policy compliance
• stackgen/mcp.json — MCP server configuration (cloud.stackgen.com/api/mcp/sse)

Capabilities

• Design, create, and manage cloud infrastructure (AWS, Azure, GCP)
• Add/update/connect cloud resources with dependency tracking
• Manage environment profiles for dev/staging/prod
• Push generated IaC to Git repositories
• Check policy violations and compliance benchmarks
• Snapshot and restore infrastructure configurations

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