feat: Add installAsUser option for user-local Claude Code installation

[Siddhant-K-code]

Summary

Adds installAsUser option to allow installing Claude Code locally for the current user instead of globally, resolving permission issues with auto-updates when running devcontainers as non-root users.

Fixes #20

Usage

"features": {
    "ghcr.io/anthropics/devcontainer-features/claude-code:1": {
        "installAsUser": true
    }
}

Benefits

• ✅ No root permissions required for Claude Code updates
• ✅ Prevents permission errors when running as vscode user
• ✅ Eliminates need to run /migrate-installer after container rebuilds
• ✅ Backward compatible (default behavior unchanged)