[alichtman]

Related to #921. I, like some others that use jrnl, have a git repo backing my jrnls up to a remote git repo. This allows me to sync my jrnls across multiple machines. It is currently a manual process, and this PR adds some infra to automate it + configure it.

This also unblocks the addition of jrnl mobile apps, which will rely on a remote git repo for syncing across devices.

Supersedes #2062 (moved from a fork branch to a branch on this repo now that I'm a maintainer here).

Summary

• Automatically commit journal changes to git when git: true is set per-journal or backup_all_jrnls_with_git is set globally. Adds GitPython dependency.
• Auto-pull from remote before reading entries with auto_pull_from_git_remote_before_edit (uses fetch + fast-forward instead of git pull to avoid unexpected merges).
• Auto-push to remote after writing with auto_push_to_git_remote_after_edit.
• Add --git/--no-git CLI flags to override git operations for a single invocation.

Test plan

• Unit tests for jrnl/git.py (init, commit, pull, push, error handling)
• Unit tests for journal write git integration (file and folder journals)
• Unit tests for git config validation
• BDD feature tests covering commit, push, pull, and --no-git flag scenarios
• Manual testing
• CI

  # 1. Lint (matches CI exactly)
  poetry run ruff check .
  poetry run black --check .

  # 2. All unit tests
  poetry run pytest tests/unit/ -x -q

  # 3. All BDD tests
  poetry run pytest tests/bdd/ -x -q

  # 4. Specifically the tests that were failing — git features
  poetry run pytest tests/unit/test_git.py tests/unit/test_journal_write_git.py tests/unit/test_parse_args.py -v

  # 5. BDD git tests (push, pull, commit, no-git)
  poetry run pytest tests/bdd/test_features.py -k "git" -v

  # 6. Upgrade tests (the square brackets flow-style bug)
  poetry run pytest tests/bdd/test_features.py -k "upgrade" -v

  # 7. Full CI-equivalent run (lint + tox)
  poetry run poe test

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.