Add --json output mode for structured data export

[dacdoyx]

Adds a --json flag that outputs the git context as a structured JSON object instead of the default human-readable format.

Closes #2

Changes

• Added --json argument to CLI parser
• When --json is passed, outputs all context data as a JSON object with keys: repo, generated_at, path, git_info, recent_commits, branches, project_structure, file_contents
• Only includes file_contents when --files flag is also set
• Existing behavior unchanged when --json is not used