feat: add --json output mode for structured AI context

[vratprem-art]

Hi there! 👋 I've implemented the $2 bounty to add a --json output flag.

🚀 What this PR does:

• Added a --json argument to both __init__.py and the standalone git-context script.
• When --json is used, the context is output as a structured JSON dictionary containing repo_name, git_info, recent_commits, project_structure, and file_contents.
• Bug Fix: Added sys.stdout.reconfigure(encoding='utf-8', errors='replace') at the start of main(). This fixes a UnicodeEncodeError that occurs on Windows consoles when the script tries to print folder/file emoji icons.
• Updated the README.md to document the new --json flag.

🧪 Testing:

Tested locally on Windows and the JSON output is perfectly formatted and valid. Let me know if you need any changes!