Add JSON output mode

[sato820]

Fixes #2.

Adds a machine-readable --json output mode for git-context while preserving the existing Markdown output.

What changed:

• Adds --json to both the standalone git-context script and the installed package entry point.
• Emits structured repo data: repo/path, git info, working tree state, commits, branches, project structure, and optional file contents.
• Keeps --output working in JSON mode and writes files as UTF-8.
• Configures CLI stdout as UTF-8 so JSON/Markdown output with tree characters works reliably on Windows terminals too.
• Documents the new --json usage in the README.

Verified locally:

• python git-context --dir . --json parses as valid JSON.
• package entry point via git_context.main() parses as valid JSON.
• python git-context --dir . --depth 1 still produces Markdown output.
• python git-context --dir . --json -o <file> writes valid JSON.

Bounty payout: BTC on-chain bc1q4zlvgnskuxmrnywk2y8klcdjf4zmkjau0e5srm. I can also provide a fresh Lightning invoice on acceptance if that is easier for the $2 payout.