Add --json output mode (Bounty: $2 crypto)

[qlufiq-collab]

Summary

Add a --json flag to git-context so the generated repo context can be consumed programmatically. With --json, the tool emits a structured JSON document instead of the human-readable Markdown block, while preserving every existing flag and code path.

Changes

• git-context — Add import json, register --json argument, refactor main() to collect git/tree/file data once and branch on --json to either emit json.dumps(..., indent=2) or the existing Markdown sections.
• git_context/__init__.py — Same change; this file is kept identical to the standalone script (it is the package entry point).

Test Plan

• python3 git-context --json | python3 -m json.tool → valid JSON with keys repo, generated, path, git, log, branches, tree.
• python3 git-context --json --files → JSON document additionally contains a files key with source contents.
• python3 git-context --json -o /tmp/ctx.json → writes valid JSON to disk; json.load succeeds.
• python3 git-context → unchanged Markdown output (default text mode is untouched).
• python3 -m git_context --json → module entry point produces the same JSON.

Notes

• Zero new dependencies; json is in the stdlib.
• JSON shape: git is an object with branch, remote, unstaged, staged, clean; log and branches are arrays of lines for easy iteration; tree and files are kept as preformatted strings to match the existing rendering.
• --json composes with --depth, --files, --log, --output, and --dir.

Closes #2