Add comprehensive testing infrastructure

This commit adds a complete testing setup for the prettier_action:

- Adds BATS (Bash Automated Testing System) testing framework
- Creates unit tests for _git_setup() and _git_changed() functions
- Creates plugin validation tests to ensure proper prettier plugin format
- Creates integration tests for end-to-end workflows
- Adds automated test runner script (tests/run_tests.sh)
- Adds GitHub Actions workflow for CI/CD testing
- Includes ShellCheck linting for bash scripts
- Updates README with comprehensive testing documentation
- Updates .gitignore to exclude test artifacts

Test coverage includes:
- Git configuration with different identity modes
- File change detection
- Plugin name validation (official, community, and scoped formats)
- Working directory handling
- node_modules cleanup
- package-lock.json restoration
- only_changed file filtering
- Dry run behavior

The test suite can be run locally with ./tests/run_tests.sh and runs
automatically on all pushes and pull requests.

tests/unit_tests.bats

@@ -0,0 +1,124 @@
+#!/usr/bin/env bats
+
+# Unit tests for prettier_action entrypoint.sh functions
+
+load 'test_helper'
+
+setup() {
+    setup_test_repo
+    mock_github_env
+    set_default_inputs
+
+    # Load the functions from entrypoint.sh
+    export SCRIPT_DIR="$(cd "$(dirname "${BATS_TEST_DIRNAME}")" && pwd)"
+    load_script_functions "$SCRIPT_DIR/entrypoint.sh"
+}
+
+teardown() {
+    teardown_test_repo
+}
+
+# Test _git_setup function with 'actions' identity
+@test "_git_setup creates .netrc file with correct permissions" {
+    export INPUT_GIT_IDENTITY="actions"
+
+    run _git_setup
+
+    [ "$status" -eq 0 ]
+    [ -f "$HOME/.netrc" ]
+
+    # Check file permissions (should be 600)
+    local perms=$(stat -c "%a" "$HOME/.netrc")
+    [ "$perms" = "600" ]
+}
+
+@test "_git_setup configures git with 'actions' identity" {
+    export INPUT_GIT_IDENTITY="actions"
+
+    run _git_setup
+
+    [ "$status" -eq 0 ]
+
+    # Check git config
+    local git_name=$(git config --global user.name)
+    local git_email=$(git config --global user.email)
+
+    [ "$git_name" = "GitHub Action" ]
+    [ "$git_email" = "actions@github.com" ]
+}
+
+@test "_git_setup configures git with 'author' identity" {
+    export INPUT_GIT_IDENTITY="author"
+    export GITHUB_ACTOR="test-user"
+    export GITHUB_ACTOR_ID="54321"
+
+    run _git_setup
+
+    [ "$status" -eq 0 ]
+
+    # Check git config
+    local git_name=$(git config --global user.name)
+    local git_email=$(git config --global user.email)
+
+    [ "$git_name" = "test-user" ]
+    [ "$git_email" = "54321+test-user@users.noreply.github.com" ]
+}
+
+@test "_git_setup fails with invalid identity" {
+    export INPUT_GIT_IDENTITY="invalid"
+
+    run _git_setup
+
+    [ "$status" -eq 1 ]
+    [[ "$output" =~ "GIT_IDENTITY must be either 'author' or 'actions'" ]]
+}
+
+@test "_git_changed returns true when files are modified" {
+    # Create and commit a file
+    echo "test" > test.txt
+    git add test.txt
+    git commit -m "Initial commit"
+
+    # Modify the file
+    echo "modified" > test.txt
+
+    run _git_changed
+
+    [ "$status" -eq 0 ]
+}
+
+@test "_git_changed returns false when no files are modified" {
+    # Create and commit a file
+    echo "test" > test.txt
+    git add test.txt
+    git commit -m "Initial commit"
+
+    # No modifications
+    run _git_changed
+
+    [ "$status" -eq 1 ]
+}
+
+@test "_git_changed returns true for untracked files" {
+    # Create a file without committing
+    echo "test" > untracked.txt
+
+    run _git_changed
+
+    [ "$status" -eq 0 ]
+}
+
+@test "_git_changed returns true for staged files" {
+    # Create and commit a file
+    echo "test" > test.txt
+    git add test.txt
+    git commit -m "Initial commit"
+
+    # Add a new file and stage it
+    echo "new file" > new.txt
+    git add new.txt
+
+    run _git_changed
+
+    [ "$status" -eq 0 ]
+}