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/unit/test_cache.fish

@@ -0,0 +1,98 @@
+#!/usr/bin/env fish
+# Unit tests for nvm_cache.fish
+
+source tests/test_runner.fish
+
+function test_cache_basic_operations
+    echo "Testing basic cache operations..."
+
+    # Test set and get
+    nvm_cache set test_key test_value
+    set -l result (nvm_cache get "test_key")
+    assert_equals "$result" test_value "Cache set and get works"
+
+    # Test delete
+    nvm_cache delete test_key
+    nvm_cache get test_key
+    set -l status_code $status
+    test $status_code -ne 0
+    and echo "✅ Cache delete works"
+    or echo "❌ Cache delete failed"
+
+    return 0
+end
+
+function test_cache_ttl
+    echo "Testing cache TTL..."
+
+    # Set with short TTL
+    nvm_cache set ttl_key ttl_value
+
+    # Should exist immediately
+    set -l result (nvm_cache get "ttl_key" 10)
+    assert_equals "$result" ttl_value "Cache value exists within TTL"
+
+    # Mock expired cache by setting TTL to 0
+    set -l result (nvm_cache get "ttl_key" 0)
+    set -l status_code $status
+    test $status_code -ne 0
+    and echo "✅ Cache TTL expiration works"
+    or echo "❌ Cache TTL expiration failed"
+
+    return 0
+end
+
+function test_cache_stats
+    echo "Testing cache stats..."
+
+    # Clear cache first
+    nvm_cache clear
+
+    # Add some items
+    nvm_cache set stats_key1 value1
+    nvm_cache set stats_key2 value2
+
+    # Get stats
+    set -l stats (nvm_cache stats)
+    assert_contains "$stats" "Cache files: 2" "Cache stats shows correct file count"
+
+    return 0
+end
+
+function test_cache_key_generation
+    echo "Testing cache key generation..."
+
+    # Test directory-based key
+    set -l key1 (_nvm_cache_key "test_file.txt")
+    set -l key2 (_nvm_cache_key "test_file.txt")
+    assert_equals "$key1" "$key2" "Same file generates same cache key"
+
+    # Test different files generate different keys
+    set -l key3 (_nvm_cache_key "different_file.txt")
+    assert_not_equals "$key1" "$key3" "Different files generate different cache keys"
+
+    return 0
+end
+
+function main
+    setup_test_env
+
+    set -l failed 0
+
+    test_cache_basic_operations; or set failed (math "$failed + 1")
+    test_cache_ttl; or set failed (math "$failed + 1")
+    test_cache_stats; or set failed (math "$failed + 1")
+    test_cache_key_generation; or set failed (math "$failed + 1")
+
+    cleanup_test_env
+
+    if test $failed -eq 0
+        echo "🎉 All cache tests passed!"
+        return 0
+    else
+        echo "💥 $failed cache test(s) failed"
+        return 1
+    end
+end
+
+main