bug: refine global gitignore and remove untracked assets

[devmocrea]

Description

Audit the root .gitignore file. Ensure all Hardhat, Clarinet, and Next.js build cache directories are excluded from git tracking.

Technical Specification

• Update .gitignore to add .turbo, cache, artifacts, and target/ exclusions.
• Clean up any previously committed cache files using git rm --cached.

Acceptance Criteria

• No temporary build files are tracked in git.