add checks for dependencies

[bruhjeshhh]

closes #9

Summary

• Add a DepsCheck to catch missing dependencies right after cloning a repo.
• Node: fail if node_modules/ is missing.
• Python: fail if neither .venv/ nor venv/ exists.
• Go: pass if vendor/ exists; otherwise run go mod download (via goModDownload) and fail if it errors.
• Wire the new check into the Go, Node, and Python stack entries in registry.go.
• Add unit tests covering pass and fail cases for each stack.

[vidya381]

merged