feat: refactor plugin architecture, enhance linting, CI & tooling

- Major refactor of core Fish functions for modularity, caching, and error handling
- Improved `.editorconfig` and Makefile for stricter formatting and linting standards
- Expanded linting support: added EditorConfig checks, auto-install for missing tools, and Makefile targets
- Enhanced CI workflow with clearer permissions and job steps in GitHub Actions
- Updated documentation in `README.md` and `CLAUDE.md` to reflect new features, advanced developer tools, and contribution guidelines
- Improved Node.js version manager detection, switching, and installation logic
- Added/updated utility functions for configuration, silent mode, notifications, and version extraction
- Various bug fixes, code quality improvements, and expanded test coverage

tests/integration/test_version_switching.fish

@@ -0,0 +1,178 @@
+#!/usr/bin/env fish
+# Integration tests for version switching functionality
+# All tests operate under $TEST_DIR (a temporary directory) for safety
+
+source tests/test_runner.fish
+
+function test_nvmrc_detection
+    echo "Testing .nvmrc file detection..."
+
+    # Create test project with .nvmrc in temp dir
+    mkdir -p "$TEST_DIR/test_project"
+    echo "18.17.0" >"$TEST_DIR/test_project/.nvmrc"
+
+    cd "$TEST_DIR/test_project"
+    set -l found_file (nvm_find_nvmrc)
+    assert_contains "$found_file" ".nvmrc" "Found .nvmrc file in current directory"
+
+    # Test parent directory search
+    mkdir -p subdir
+    cd subdir
+    set found_file (nvm_find_nvmrc)
+    assert_contains "$found_file" ".nvmrc" "Found .nvmrc file in parent directory"
+
+    cd "$TEST_DIR"
+    rm -rf "$TEST_DIR/test_project"
+
+    return 0
+end
+
+function test_version_extraction
+    echo "Testing version extraction from different file formats..."
+
+    cd "$TEST_DIR"
+    # Test .nvmrc
+    echo "18.17.0" >test.nvmrc
+    set -l version (nvm_extract_version "test.nvmrc")
+    assert_equals "$version" "18.17.0" "Extracted version from .nvmrc"
+
+    # Test .node-version
+    echo "16.20.0" >test.node-version
+    set version (nvm_extract_version "test.node-version")
+    assert_equals "$version" "16.20.0" "Extracted version from .node-version"
+
+    # Test .tool-versions
+    echo "nodejs 20.5.0" >test.tool-versions
+    set version (nvm_extract_version "test.tool-versions:nodejs")
+    assert_equals "$version" "20.5.0" "Extracted version from .tool-versions"
+
+    # Test package.json (requires jq)
+    if command -q jq
+        echo '{"engines": {"node": ">=18.0.0"}}' >test.package.json
+        set version (nvm_extract_version "test.package.json:engines.node")
+        assert_equals "$version" "18.0.0" "Extracted version from package.json"
+    else
+        echo "ℹ️  Skipping package.json test (jq not available)"
+    end
+
+    # Cleanup
+    rm -f test.nvmrc test.node-version test.tool-versions test.package.json
+
+    return 0
+end
+
+function test_manager_detection
+    echo "Testing version manager detection..."
+
+    cd "$TEST_DIR"
+    set -l managers (nvm_compat_detect)
+
+    if test -n "$managers"
+        echo "✅ Found version managers: $managers"
+    else
+        echo "ℹ️  No version managers found (expected in test environment)"
+    end
+
+    return 0
+end
+
+function test_error_recovery
+    echo "Testing error recovery mechanisms..."
+
+    cd "$TEST_DIR"
+    # Test invalid version handling
+    echo "invalid.version" >invalid.nvmrc
+    set -l result (nvm_extract_version "invalid.nvmrc" 2>/dev/null)
+
+    if test -z "$result"
+        echo "✅ Invalid version file handled gracefully"
+    else
+        echo "❌ Invalid version should return empty result"
+    end
+
+    # Test missing file handling
+    nvm_extract_version "nonexistent.nvmrc" >/dev/null 2>&1
+    set -l status_code $status
+    test $status_code -ne 0
+    and echo "✅ Missing file handled gracefully"
+    or echo "❌ Missing file should return error"
+
+    rm -f invalid.nvmrc
+
+    return 0
+end
+
+function test_async_operations
+    echo "Testing async operations..."
+
+    cd "$TEST_DIR"
+    # Create test version file
+    echo "18.17.0" >async_test.nvmrc
+
+    # Test async version check
+    set -l job_id (nvm_async version_check "async_test.nvmrc")
+
+    if test -n "$job_id"
+        echo "✅ Async version check started"
+
+        # Wait for completion
+        nvm_async wait "$job_id" 5
+        and echo "✅ Async operation completed"
+        or echo "⚠️  Async operation timed out"
+    else
+        echo "ℹ️  Async operation may have completed immediately"
+    end
+
+    rm -f async_test.nvmrc
+
+    return 0
+end
+
+function test_cache_integration
+    echo "Testing cache integration..."
+
+    cd "$TEST_DIR"
+    # Clear cache first
+    nvm_cache clear
+
+    # Create test file
+    echo "18.17.0" >cache_test.nvmrc
+
+    # First access should miss cache
+    set -l start_time (date +%s)
+    set -l version1 (nvm_extract_version "cache_test.nvmrc")
+
+    # Second access should hit cache (if caching is implemented)
+    set -l version2 (nvm_extract_version "cache_test.nvmrc")
+
+    assert_equals "$version1" "$version2" "Consistent results from cache"
+
+    rm -f cache_test.nvmrc
+
+    return 0
+end
+
+function main
+    setup_test_env
+
+    set -l failed 0
+
+    test_nvmrc_detection; or set failed (math "$failed + 1")
+    test_version_extraction; or set failed (math "$failed + 1")
+    test_manager_detection; or set failed (math "$failed + 1")
+    test_error_recovery; or set failed (math "$failed + 1")
+    test_async_operations; or set failed (math "$failed + 1")
+    test_cache_integration; or set failed (math "$failed + 1")
+
+    cleanup_test_env
+
+    if test $failed -eq 0
+        echo "🎉 All integration tests passed!"
+        return 0
+    else
+        echo "💥 $failed integration test(s) failed"
+        return 1
+    end
+end
+
+main