docs: add CONTRIBUTING.md, fix Makefile, and improve .gitignore

[Dakshx07]

📝 Overview

This PR significantly improves the Developer Experience (DX) for new contributors. While setting up the project, I identified several "friction points" that could discourage new developers. This contribution solves those issues at the foundation.

🛠️ Key Improvements

1. New CONTRIBUTING.md

Added a comprehensive guide for new developers.

• Lists all polyglot prerequisites (Bun, Rust, Go).
• Explains the monorepo structure (App vs Engine).
• Provides a clear Quick Start using the Makefile.

2. Makefile Logic Fixes

• Resolved Build Crash: Fixed a bug where make install would fail because it attempted to enter a non-existent sandman/ directory.
• Improved Installation Flow: Added pty-dev to the install target. This ensures that the Rust-based terminal engine is built automatically during the initial setup.

3. Repository Hygiene (.gitignore)

• Optimized the root .gitignore to recursively ignore node_modules/.
• This prevents noise in source control and keeps the repo clean.

📸 Visual Proof

Before (Build Failure)

Caution

make install fails with /bin/sh: line 0: cd: sandman: No such file or directory

After (Successful Setup)

Tip

make install now completes all tasks across the monorepo in one go.

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🏁 Final Check

• Tested make install on local machine
• Verified CONTRIBUTING.md formatting
• Confirmed no unnecessary files are being tracked by Git