fix: detect if git is installed and exit with a clear error

[sunasrd-byte]

Changes:

• git_preflight.py (new): require_git() reports a clean message and exits 1.
• plain_modules.py: call require_git() at module scope, immediately above the
  GitPython import.
• .flake8: E402 per-file ignore for plain_modules.py — require_git() is a
  statement between imports, so every import below it trips E402.
• tests/test_git_preflight.py (new): subprocess tests asserting a clean
  message, no traceback, and exit 1 for --version, --status, and a render,
  under both a git-less and a broken-git PATH.

Two failure modes are diagnosed separately:

• git missing — nothing named git on PATH.
• git present but broken — resolves but cannot run 'git version'. This is the
  normal state on macOS without the Command Line Tools, where /usr/bin/git is
  a stub that exits with an xcrun error. GIT_PYTHON_REFRESH=quiet does not
  suppress this: GitPython only consults that setting for a git it cannot
  find. A PATH lookup alone is therefore insufficient, so require_git runs
  'git version' — matching git_available in install/bash/install.sh.

Verified in python:3.12-slim without git, per the issue's reproduction steps.

Note: GitNotInstalledError is now unraised. Left in place rather than removed
as a drive-by API change.

Closes #133