Instead of resetting GIT_CONFIG_COUNT to 1, read the existing count
from the environment and append safe.directory at the next available
index. This preserves any env-scoped git config (auth, URL rewrites,
extra headers) injected into the daemon process.
Adds TestGitEnvPreservesExistingConfig to verify the append behavior.
Co-authored-by: multica-agent <github@multica.ai>