test: add BATS test suite with 50 tests

Covers arg parsing, version comparison, audit phase (git config,
signing, SSH), apply phase (settings, SSH directives, url rewrite),
signing key detection (standard/custom/tilde/sk-preference),
allowed signers, -y mode, backup, and end-to-end idempotency.
All tests run in isolated HOME to avoid touching real config.

Closes: #6

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/run.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+# Run the BATS test suite
+set -o errexit
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+"${SCRIPT_DIR}/libs/bats-core/bin/bats" "${SCRIPT_DIR}/git-harden.bats" "$@"